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: https://reviews.llvm.org/D102155