[CMake][AIX] Set LLVM_ENABLE_PER_TARGET_RUNTIME_DIR appropriately for AIX

AIX by default usually folds 32-bit & 64-bit arch libraries into a single
archive, a behaviour we may want for the runtime libraries in the future,
so we don't necessarily want to opt into the multlib layout introduce in
D45604, which is currently the default for runtime builds.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D88169