[compiler-rt] [FMV][AArch64] Add initial AT_HWCAP3 / AT_HWCAP4 support (PR #161595)

[Paul Kirth via llvm-commits]

ilovepi wrote:

We're seeing some build failures after this patch. It seems you're assuming there is some minium libc version you can assume, but AFAIK there is no such minimum stated in LLVM. I think libcxx has some minimum, but that's very much separate from compiler-rt and the builtins. I think this should probably be reverted until there's some community agreement about updating (in this case specifying at all) what's required.

```console
[273/417](25) Building C object CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o
FAILED: CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o 
/Users/swarming/b/s/w/ir/x/w/llvm_build/./bin/clang --target=aarch64-unknown-linux-gnu --sysroot=/Users/swarming/b/s/w/ir/x/w/cipd/linux -DHAS_ASM_LSE -DVISIBILITY_HIDDEN -D_LIBATOMIC_USE_PTHREAD -I/Users/swarming/b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/../../../third-party/siphash/include --target=aarch64-unknown-linux-gnu -O2 -g -DNDEBUG -std=gnu11 -fno-lto -nostdinc++ -fPIC -fno-builtin -fvisibility=hidden -fomit-frame-pointer -DCOMPILER_RT_HAS_FLOAT16 -MD -MT CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o -MF CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o.d -o CMakeFiles/clang_rt.builtins-aarch64.dir/cpu_model/aarch64.c.o -c /Users/swarming/b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64.c
In file included from /Users/swarming/b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64.c:79:
/Users/swarming/b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/lib/builtins/cpu_model/aarch64/fmv/getauxval.inc:19:35: error: use of undeclared identifier 'hwcap'
   19 |   __init_cpu_features_constructor(hwcap | _IFUNC_ARG_HWCAP, &arg);
      |                                   ^~~~~
1 error generated.
```

Bot: https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci/clang-mac-arm64/b8701635553374622177/overview

Logs: https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8701635553374622177/+/u/clang/build/stdout

https://github.com/llvm/llvm-project/pull/161595