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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

musl makefiles have an "install-headers" target, and that's the officially endorsed way to build a cross toolchain from scratch (see https://github.com/richfelker/musl-cross-make/blob/master/litecross/Makefile).  I assume other libcs have something similar.

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