[CMake][AIX] Add `-bcdtors:mbr` option when building with IBM XL

Summary:
The `-bcdtors:mbr` option causes processing for constructors and
destructors to omit otherwise-unreferenced members of static libraries,
matching the processing done on Linux, where `--whole-archive` is not
the default. Applying this option is desirable for reducing the
footprint of an installation.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D79749