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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

Thank you for informing.

My understand of a typical [nixpkgs DAG](https://github.com/NixOS/nixpkgs/blob/e62ef354ddcaa7827bcc8c7adcaf863304b70158/pkgs/development/compilers/llvm/common/default.nix#L1111) is effectively something like:
```
() -> compiler-rt-nolibc
() -> libc
compiler-rt-no-libc -> libcxx
libc -> libcxx
```

Where compiler-rt-no-libc is defined to be a baremetal build. That is, by the time libcxx is linked, libc is available.

I can believe this is in some sense wrong and that it should use a different method (e.g. making the headers available). There is evidence for this in that it does have to remove a couple of `#include <assert.h>` headers for example which are provided by libc, but the edits appear to be minimal: https://github.com/NixOS/nixpkgs/blob/16c225539220d31bee2f5696b22853504452708f/pkgs/development/compilers/llvm/common/compiler-rt/default.nix#L163-L166

On the other hand I understand this has worked in this way for a long while, not to suggest that this means it is correct. Do you think there is scope to reinstate compiler-rt to as it was before https://github.com/llvm/llvm-project/pull/119414, or must nixpkgs switch things around to make the libc headers available to compiler-rt?

Lastly, is it known that the builtins are unnecessary to build libc(s), are those typically built with -fno-builtins or similar?

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