Auto merge of #127897 - nyurik:add-qnx-70-target, r=saethlin

add `aarch64_unknown_nto_qnx700` target - QNX 7.0 support for aarch64le

This backports the QNX 7.1 aarch64 implementation to 7.0.

* [x] required `-lregex` disabled, see https://github.com/rust-lang/libc/pull/3775 (released in libc 0.2.156)
* [x] uses `libgcc.a` instead of `libgcc_s.so` (7.0 used ancient GCC 5.4 which didn't have gcc_s)
* [x] a fix in `backtrace` crate to support stack traces https://github.com/rust-lang/backtrace-rs/pull/648

This PR bumps libc dependency to 0.2.158

CC: to the folks who did the [initial implementation](https://doc.rust-lang.org/rustc/platform-support/nto-qnx.html): `@flba-eb,` `@gh-tr,` `@jonathanpallant,` `@japaric`

# Compile target

```bash
# Configure qcc build environment
source _path_/_to_/qnx7.0/qnxsdp-env.sh

# Tell rust to use qcc when building QNX 7.0 targets
export build_env='
    CC_aarch64-unknown-nto-qnx700=qcc
    CFLAGS_aarch64-unknown-nto-qnx700=-Vgcc_ntoaarch64le_cxx
    CXX_aarch64-unknown-nto-qnx700=qcc
    AR_aarch64_unknown_nto_qnx700=ntoaarch64-ar'

# Build rust compiler, libs, and the remote test server
env $build_env ./x.py build \
  --target x86_64-unknown-linux-gnu,aarch64-unknown-nto-qnx700 \
  rustc library/core library/alloc library/std src/tools/remote-test-server

rustup toolchain link stage1 build/host/stage1
```

# Compile "hello world"

```bash
source _path_/_to_/qnx7.0/qnxsdp-env.sh

cargo new hello_world
cd hello_world
cargo +stage1 build --release --target aarch64-unknown-nto-qnx700
```

# Configure a remote for testing

Do this from a new shell - we will need to run more commands in the previous one.  I ran into these two issues, and found some workarounds.

* Temporary dir might not work properly
* Default `remote-test-server` has issues binding to an address

```
# ./remote-test-server
starting test server
thread 'main' panicked at src/tools/remote-test-server/src/main.rs:175:29:
called `Result::unwrap()` on an `Err` value: Os { code: 249, kind: AddrNotAvailable, message: "Can't assign requested address" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Specifying `--bind` param actually fixes that, and so does setting `TMPDIR` properly.

```bash
# Copy remote-test-server to remote device. You may need to use sftp instead.
# ATTENTION: Note that the path is different from the one in the remote testing documentation for some reason
scp ./build/x86_64-unknown-linux-gnu/stage1-tools-bin/remote-test-server  qnxdevice:/path/

# Run ssh with port forwarding - so that rust tester can connect to the local port instead
ssh -L 12345:127.0.0.1:12345 qnxdevice

# on the device, run
rm -rf tmp && mkdir -p tmp && TMPDIR=$PWD/tmp ./remote-test-server --bind 0.0.0.0:12345
```

# Run test suit

Assume all previous environment variables are still set, or re-init them

```bash
export TEST_DEVICE_ADDR="localhost:12345"

# tidy needs to be skipped due to using un-published libc dependency
export exclude_tests='
    --exclude src/bootstrap
    --exclude src/tools/error_index_generator
    --exclude src/tools/linkchecker
    --exclude src/tools/tidy
    --exclude tests/ui-fulldeps
    --exclude rustc
    --exclude rustdoc
    --exclude tests/run-make-fulldeps'

env $build_env ./x.py test  $exclude_tests --stage 1 --target aarch64-unknown-nto-qnx700
```

try-job: dist-x86_64-msvc

2 files changed, 19 insertions, 0 deletions

diff --git a/src/doc/rustc/src/platform-support.md b/src/doc/rustc/src/platform-support.md
index 0d79e5f4114..a2641b22753 100644
--- a/src/doc/rustc/src/platform-support.md
+++ b/src/doc/rustc/src/platform-support.md
@@ -256,6 +256,7 @@ target | std | host | notes
 [`aarch64-kmc-solid_asp3`](platform-support/kmc-solid.md) | ✓ |  | ARM64 SOLID with TOPPERS/ASP3
 [`aarch64-nintendo-switch-freestanding`](platform-support/aarch64-nintendo-switch-freestanding.md) | * |  | ARM64 Nintendo Switch, Horizon
 [`aarch64-unknown-teeos`](platform-support/aarch64-unknown-teeos.md) | ? |  | ARM64 TEEOS |
+[`aarch64-unknown-nto-qnx700`](platform-support/nto-qnx.md) | ? |  | ARM64 QNX Neutrino 7.0 RTOS |
 [`aarch64-unknown-nto-qnx710`](platform-support/nto-qnx.md) | ✓ |  | ARM64 QNX Neutrino 7.1 RTOS |
 `aarch64-unknown-freebsd` | ✓ | ✓ | ARM64 FreeBSD
 [`aarch64-unknown-hermit`](platform-support/hermit.md) | ✓ |  | ARM64 Hermit
diff --git a/src/doc/rustc/src/platform-support/nto-qnx.md b/src/doc/rustc/src/platform-support/nto-qnx.md
index 51a397a38d2..1c240d1255a 100644
--- a/src/doc/rustc/src/platform-support/nto-qnx.md
+++ b/src/doc/rustc/src/platform-support/nto-qnx.md
@@ -24,6 +24,7 @@ Currently, the following QNX Neutrino versions and compilation targets are suppo
 |----------------------|---------------------|:------------:|:----------------:|
 | 7.1 | AArch64 | ✓ | ✓ |
 | 7.1 | x86_64  | ✓ | ✓ |
+| 7.0 | AArch64 | ? | ✓ |
 | 7.0 | x86     |   | ✓ |
 
 Adding other architectures that are supported by QNX Neutrino is possible.
@@ -43,6 +44,23 @@ When linking `no_std` applications, they must link against `libc.so` (see exampl
 required because applications always link against the `crt` library and `crt` depends on `libc.so`.
 This is done automatically when using the standard library.
 
+### Disabling RELocation Read-Only (RELRO)
+
+While not recommended by default, some QNX kernel setups may require the `RELRO` to be disabled with `-C relro_level=off`, e.g. by adding it to the `.cargo/config.toml` file:
+
+```toml
+[target.aarch64-unknown-nto-qnx700]
+rustflags = ["-C", "relro_level=off"]
+```
+
+If your QNX kernel does not allow it, and `relro` is not disabled, running compiled binary would fail with `syntax error: ... unexpected` or similar.  This is due to kernel trying to interpret compiled binary with `/bin/sh`, and obviously failing.  To verify that this is really the case, run your binary with the `DL_DEBUG=all` env var, and look for this output. If you see it, you should disable `relro` as described above.
+
+```text
+Resolution scope for Executable->/bin/sh:
+        Executable->/bin/sh
+        libc.so.4->/usr/lib/ldqnx-64.so.2
+```
+
 ### Small example application
 
 Small `no_std` example is shown below. Applications using the standard library work as well.