Auto merge of #99464 - nikic:llvm-15, r=cuviper

Update to LLVM 15

For preliminary testing. Some LLVM 15 compatibility fixes were applied separately in #99512.

Release timeline:
 * LLVM 15 branched on Jul 26.
 * The final LLVM 15.0.0 release is scheduled for Sep 6.
 * Current nightly (1.65.0) is scheduled for Nov 3.

Changes in this PR (apart from the LLVM update):
 * Pass `--set llvm.allow-old-toolchain` for many Docker images. LLVM 16 will require GCC >= 7.1, while LLVM 15 still allows older compilers with an option. Specify the option for builders still using GCC 5.4. #95026 updated some of the used toolchains, but not all.
 * Use the `+atomics-32` target feature for thumbv6m.
 * Explicitly link libatomic when cross-compiling LLVM to 32-bit target.
 * Explicitly disable zstd support, to avoid libzstd.so dependency.

New LLVM patches ([commits](https://github.com/rust-lang/llvm-project/commits/rustc/15.0-2022-08-09)):
 * [rust-only] Fix ICE with GCC 5.4 (https://github.com/nikic/llvm-project/commit/15be58d7f0342b1da5af219bac8bd71d01da6dff)
 * [rust-only] Fix build with GCC 5.4 (https://github.com/nikic/llvm-project/commit/774edc10fa45229c2aa678f1bef8b4812dc0f76a)
 * ~~[rust-only] Fix build with GCC 5.2 (https://github.com/nikic/llvm-project/commit/1a6069a7bb35ace1e40d566035cbf7ed2fa3b1f7)~~
 * ~~[rust-only] Fix ICE with GCC 5.2 (https://github.com/nikic/llvm-project/commit/493081f2909206e0ed55af68a4058a76c0ad7a64)~~
 * ~~[rust-only] Fix build with GCC 5.2 (https://github.com/nikic/llvm-project/commit/0fc5979d738c3a1f9510fe2d62417f7d2af37817)~~
 * [backported] Addition of `+atomics` target feature (https://github.com/llvm/llvm-project/commit/57bdd9892d0eba5bdd25fc44799235be7b9f5153).
 * [backported] Revert compiler-rt change that broke powerpc (https://github.com/llvm/llvm-project/commit/9c68b43915fc1c9c0a07e935163ae8d638d7241b)
 * [awaiting backport] Fix RelLookupTableConverter on gnux32 (https://github.com/nikic/llvm-project/commit/639388a05f25772fb23eea5b1045e7df83bcfaa7 / https://github.com/llvm/llvm-project/issues/57021)

Tested images: dist-x86_64-linux, armhf-gnu, arm-android, dist-s390x-linux, dist-x86_64-illumos, dist-x86_64-freebsd, wasm32, dist-x86_64-musl, dist-various-1, dist-riscv64-linux, dist-mips-linux, dist-mipsel-linux, dist-powerpc-linux, dist-aarch64-linux, dist-x86_64-apple, x86_64-msvc-1, x86_64-msvc-2, dist-various-2, dist-arm-linux
Tested up to the usual ipv6 error: test-various, i686-gnu, x86_64-gnu-nopt

r? `@ghost`

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