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

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

It was a challenge to find this documented anywhere. I did get it working for musl via https://www.openwall.com/lists/musl/2018/03/01/8

This appears to do the trick:
```
make DESTDIR=$PWD/headers/ includedir=include ARCH=aarch64 install-headers
```

The closest I found for glibc was https://clfs.org/view/CLFS-1.0.0/alpha/cross-tools/glibc-headers.html which suggests the procedure for glibc won't be so straightforward and that it would require a compiler, potentially creating a bootstrap problem again. Any thoughts? I still find myself coming back to 'a libc free compiler-rt holds a certain appeal' as a way to solve this problem.

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