[AArch64] Compiler-rt interface for out-of-line atomics.

Out-of-line helper functions to support LSE deployment added.
This is a port of libgcc implementation:
https://gcc.gnu.org/git/?p=gcc.git;h=33befddcb849235353dc263db1c7d07dc15c9faa

Differential Revision: https://reviews.llvm.org/D91156