[libc++][CI] Add AIX pipeline config

This changes adds the pipeline config for both 32-bit and 64-bit AIX targets. As well, we add a lit feature `LIBCXX-AIX-FIXME` which is used to mark the failing tests which remain to be investigated on AIX, so that the CI produces a clean build.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D111359