[llvm-ar][test][AIX] Unsupport error-opening-directory.test on AIX

Summary:
The test fails on AIX as it allows open() and read() on a directory. This patch adds `# UNSUPPORTED:
system-aix` to the test to prevent it from running on AIX.

Reviewers: sameerarora101, daltenty, ShuhongL, hubert.reinterpretcast, MaskRay, smeenai, alexshap

Reviewed By: sameerarora101, hubert.reinterpretcast, MaskRay, smeenai

Subscribers: MaskRay, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83579