[builtins][AArch64] Implement _sync out-of-line atomics

Whilst Clang does not use these, recent GCC does, and so on systems such
as FreeBSD that wish to use compiler-rt as the system runtime library
but also wish to support building programs with GCC these interfaces are
needed.

This is a light adaptation of the code committed to GCC by Sebastian Pop
<spop@amazon.com>, relicensed with permission for use in compiler-rt.

Fixes https://github.com/llvm/llvm-project/issues/63483

Reviewed By: sebpop, MaskRay

Differential Revision: https://reviews.llvm.org/D158536