[compiler-rt][AIX] Add CMake support for 32-bit Power builds

This patch enables support for building compiler-rt builtins for 32-bit
Power arch on AIX. For now, we leave out the specialized ppc builtin
implementations for 128-bit long double and friends since those will
need some special handling for AIX.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D87383