Auto merge of #44779 - tjkirch:master, r=alexcrichton

Add aarch64-unknown-linux-musl target

This adds support for the aarch64-unknown-linux-musl target in the build and CI systems.

This addresses half of issue #42520.

The new file `aarch64_unknown_linux_musl.rs` is a copy of `aarch64_unknown_linux_gnu.rs` with "gnu" replaced by "musl", and the added logic in `build-arm-musl.sh` is similarly a near-copy of the arches around it, so overall the changes were straightforward.

Testing:

```
$ sudo ./src/ci/docker/run.sh cross
...
Dist std stage2 (x86_64-unknown-linux-gnu -> aarch64-unknown-linux-musl)
Building stage2 test artifacts (x86_64-unknown-linux-gnu -> aarch64-unknown-linux-musl)
   Compiling getopts v0.2.14
   Compiling term v0.0.0 (file:///checkout/src/libterm)
   Compiling test v0.0.0 (file:///checkout/src/libtest)
    Finished release [optimized] target(s) in 16.91 secs
Copying stage2 test from stage2 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu / aarch64-unknown-linux-musl)
...
Build completed successfully in 0:55:22
```

```
$ rustup toolchain link local obj/build/x86_64-unknown-linux-gnu/stage2
$ rustup default local
```

After setting the local toolchain as default, and adding this in ~/.cargo/config:

```
[target.aarch64-unknown-linux-musl]
linker = "aarch64-linux-musl-gcc"
```

...then the toolchain was able to build a working ripgrep as a test:

```
$ readelf -a target/aarch64-unknown-linux-musl/debug/rg | grep -i interpreter
$ readelf -a target/aarch64-unknown-linux-musl/debug/rg | grep NEEDED
$ file target/aarch64-unknown-linux-musl/debug/rg
target/aarch64-unknown-linux-musl/debug/rg: ELF 64-bit LSB executable, ARM aarch64, version 1 (GNU/Linux), statically linked, BuildID[sha1]=be11036b0988fac5dccc9f6487eb780b05186582, not stripped
```

2 files changed, 31 insertions, 1 deletions

diff --git a/src/ci/docker/cross/Dockerfile b/src/ci/docker/cross/Dockerfile
index a83bbe9c60e..05745709a07 100644
--- a/src/ci/docker/cross/Dockerfile
+++ b/src/ci/docker/cross/Dockerfile
@@ -14,6 +14,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   zlib1g-dev \
   g++-arm-linux-gnueabi \
   g++-arm-linux-gnueabihf \
+  g++-aarch64-linux-gnu \
   gcc-sparc64-linux-gnu \
   libc6-dev-sparc64-cross \
   bzip2 \
@@ -46,6 +47,7 @@ ENV TARGETS=$TARGETS,mipsel-unknown-linux-musl
 ENV TARGETS=$TARGETS,arm-unknown-linux-musleabi
 ENV TARGETS=$TARGETS,arm-unknown-linux-musleabihf
 ENV TARGETS=$TARGETS,armv7-unknown-linux-musleabihf
+ENV TARGETS=$TARGETS,aarch64-unknown-linux-musl
 ENV TARGETS=$TARGETS,sparc64-unknown-linux-gnu
 ENV TARGETS=$TARGETS,x86_64-unknown-redox
 
@@ -62,7 +64,8 @@ ENV RUST_CONFIGURE_ARGS \
       --target=$TARGETS \
       --musl-root-arm=/usr/local/arm-linux-musleabi \
       --musl-root-armhf=/usr/local/arm-linux-musleabihf \
-      --musl-root-armv7=/usr/local/armv7-linux-musleabihf
+      --musl-root-armv7=/usr/local/armv7-linux-musleabihf \
+      --musl-root-aarch64=/usr/local/aarch64-linux-musl
 ENV SCRIPT python2.7 ../x.py dist --target $TARGETS
 
 # sccache
diff --git a/src/ci/docker/cross/build-arm-musl.sh b/src/ci/docker/cross/build-arm-musl.sh
index 938e69834e4..780099e2ec1 100755
--- a/src/ci/docker/cross/build-arm-musl.sh
+++ b/src/ci/docker/cross/build-arm-musl.sh
@@ -65,11 +65,24 @@ CFLAGS="-march=armv7-a" \
 hide_output make -j$(nproc)
 hide_output make install
 cd ..
+rm -rf musl-$MUSL
+
+tar xf musl-$MUSL.tar.gz
+cd musl-$MUSL
+CC=aarch64-linux-gnu-gcc \
+CFLAGS="" \
+    hide_output ./configure \
+        --prefix=/usr/local/aarch64-linux-musl \
+        --enable-wrapper=gcc
+hide_output make -j$(nproc)
+hide_output make install
+cd ..
 rm -rf musl-$MUSL*
 
 ln -nsf ../arm-linux-musleabi/bin/musl-gcc /usr/local/bin/arm-linux-musleabi-gcc
 ln -nsf ../arm-linux-musleabihf/bin/musl-gcc /usr/local/bin/arm-linux-musleabihf-gcc
 ln -nsf ../armv7-linux-musleabihf/bin/musl-gcc /usr/local/bin/armv7-linux-musleabihf-gcc
+ln -nsf ../aarch64-linux-musl/bin/musl-gcc /usr/local/bin/aarch64-unknown-linux-musl-gcc
 
 curl -L https://github.com/llvm-mirror/llvm/archive/release_39.tar.gz | tar xzf -
 curl -L https://github.com/llvm-mirror/libunwind/archive/release_39.tar.gz | tar xzf -
@@ -116,5 +129,19 @@ cp lib/libunwind.a /usr/local/armv7-linux-musleabihf/lib
 cd ..
 rm -rf libunwind-build
 
+mkdir libunwind-build
+cd libunwind-build
+cmake ../libunwind-release_39 \
+          -DLLVM_PATH=/tmp/llvm-release_39 \
+          -DLIBUNWIND_ENABLE_SHARED=0 \
+          -DCMAKE_C_COMPILER=aarch64-linux-gnu-gcc \
+          -DCMAKE_CXX_COMPILER=aarch64-linux-gnu-g++ \
+          -DCMAKE_C_FLAGS="" \
+          -DCMAKE_CXX_FLAGS=""
+make -j$(nproc)
+cp lib/libunwind.a /usr/local/aarch64-linux-musl/lib
+cd ..
+rm -rf libunwind-build
+
 rm -rf libunwind-release_39
 rm -rf llvm-release_39