1. [AIX][compiler-rt] Build and install standalone libatomic (details)
Commit d56729b4a4393da9c65bdfe762b51f8b7b0ce0ca by lkail
[AIX][compiler-rt] Build and install standalone libatomic

On AIX, we have to ship `libatomic.a` for compatibility. First, a new `clang_rt.atomic` is added. Second, use added cmake modules for AIX, we are able to build a compatible libatomic.a for AIX. The second step can't be perfectly implemented with cmake now since AIX's archive approach is kinda unique, i.e., archiving shared libraries into a static archive file.

Reviewed By: jsji

Differential Revision:
The file was addedcompiler-rt/cmake/Modules/CompilerRTAIXUtils.cmake
The file was modifiedcompiler-rt/lib/builtins/CMakeLists.txt
The file was addedcompiler-rt/lib/builtins/ppc/atomic.exp