[AIX][cmake] Set atomics related macros when build with xlclang

Set `HAVE_CXX_ATOMICS_WITHOUT_LIB` or `HAVE_LIBATOMIC` when build LLVM with xlclang. With these macros set, libraries like libLLVMSupport are able to know whether it's necessary to add `-latomic` to dependent system libs. If `HAVE_LIBATOMIC` is set, `llvm-config --system-libs` appends `-latomic` to its output.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D111782