[Flang] Xfail hlfir test case on AIX (#76802)

This test case seems to fail at the `Merge disjoint stack slots` pass on
AIX, it passes if compilled with `-mllvm --no-stack-coloring`. This PR
xfails the teest case on AIX temporarily until the issue is addressed.

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Co-authored-by: Mark Danial <mark.danial@ibm.com>