[GlobalISel][AArch64] Fix pcsections for expanded atomics and add more tests

Add fix for propagation of !pcsections metadata for expanded atomics,
together with more tests for interesting atomic instructions (based on
llvm/test/CodeGen/AArch64/GlobalISel/arm64-atomic.ll).

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D133710