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 (15be58d7f0)
* [rust-only] Fix build with GCC 5.4 (774edc10fa)
* ~~[rust-only] Fix build with GCC 5.2 (1a6069a7bb)~~
* ~~[rust-only] Fix ICE with GCC 5.2 (493081f290)~~
* ~~[rust-only] Fix build with GCC 5.2 (0fc5979d73)~~
* [backported] Addition of `+atomics` target feature (57bdd9892d).
* [backported] Revert compiler-rt change that broke powerpc (9c68b43915)
* [awaiting backport] Fix RelLookupTableConverter on gnux32 (639388a05f / 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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