[AIX][compiler-rt] Fix cmake build of libatomic for cmake-3.16+

cmake-3.16+ for AIX changes the default behavior of building a `SHARED` library which breaks AIX's build of libatomic, i.e., cmake-3.16+ builds `SHARED` as an archive of dynamic libraries. To fix it, we have to build `libatomic.so.1` as `MODULE` which keeps `libatomic.so.1` as an normal dynamic library.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D103786