[NFC][AIX]Disable new pcm tests on AIX

Same as D114481, the PCH reader looks for a `__clangast` section in the precompiled module file, which isn't present on AIX, and we don't support writing this custom section in XCOFF yet.

Reviewed By: Jake-Egan, daltenty, sfertile

Differential Revision: https://reviews.llvm.org/D118477