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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

There's a world on some targets where you can build a "baremetal" builtins library, and have it be sufficient for some non-baremetal uses.  And it might be possible to rewrite the code in some cases so it doesn't depend on libc headers, even if it does depend on libc, by redeclaring stuff from libc.

However, neither of those has ever been a supported configuration, and I'm not sure it's worth taking the time to try to support.  And looking at the code, I think it's hard to implement correctly for non-glibc targets.

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