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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

My understanding of the way building a toolchain from scratch is supposed to work:

- libc headers are installed somewhere.
- builtins library is built against libc headers
- libc is built, linking in builtins library
- build everything else

You do it in that order to resolve the circular dependency between builtins and libc.

Given that process, I can't see how you get a link error building libc++abi; by the time you get around to building libc++abi, you should already have a newly built libc with support for the relevant routines.  If you're not doing it using that process... where is your libc coming from?

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