[compiler-rt] [compiler-rt][AArch64] Enable libc-free builtins Linux build (PR #125922)

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

Thanks again for the input. Can I ask what you mean by 'supported' - that it works out of the box, or that we promise to keep it working, or something else?

I note that it has almost worked out of the box (modulo one or two #includes which it appears could be dropped) until this change.

Considering only the issue related to this change, the problem is to provide access to hardware feature tests. AFAICT this *requires* cooperating with the libc (which owns knowledge of where the auxv is which is acquired on startup). In practice means calling the `getauxval` provided by the libc. Potentially, the prototype for getauxval could be specified and the feature constants could from the linux headers, rather than taking them from libc. This seems OK under the belief that the prototype for getauxval is identical across `libc`s.

As opposed to what this closed PR does, which is to fall through to a feature test which always returns 'no features', which I currently believe is a safe fallback; and that this is reasonable because a 'full' compiler-rt is built later and used for workloads in deployment.

This implies providing the linux headers rather than libc headers, which is already the case on nixpkgs for the nolibc linux build.

https://github.com/llvm/llvm-project/pull/125922