[AIX] Skip unsupported ORC JIT RemoteObjectLayerTest on AIX

Summary:
Currently the XCOFF backend does not support writing 64-bit object
files, which the ORC JIT tests will try to exercise if we are on AIX. This patch
selectively disables the tests on AIX for now.

Reviewers: hubert.reinterpretcast, jasonliu, DiggerLin, stevewan, lhames

Reviewed By: hubert.reinterpretcast, lhames

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78813