[AArch64] Out-of-line atomics (-moutline-atomics) implementation.

[AArch64] Out-of-line atomics (-moutline-atomics) implementation.

This patch implements out of line atomics for LSE deployment
mechanism. Details how it works can be found in llvm/docs/Atomics.rst
Options -moutline-atomics and -mno-outline-atomics to enable and disable it
were added to clang driver. This is clang and llvm part of out-of-line atomics
interface, library part is already supported by libgcc. Compiler-rt
support is provided in separate patch.

Differential Revision: https://reviews.llvm.org/D91157