[AIX][clang-repl][test] Mark unsupported pending XCOFF64 integrated-as

This patch replaces the `powerpc64` token with the `system-aix` one in
the UNSUPPORTED line of a test. The `powerpc64` token was originally
added temporarily in 71a0609a.

If AIX uses integrated-as by default and it works both for 32-bit and
64-bit objects, then the issues encountered so far (see comments in
D96033) would be mostly solved.

As it is, marking the test as expected-to-fail (as opposed to
unsupported) on AIX might cause more trouble in the form of 32-bit
versus 64-bit differences. I am not aware of other situations where LIT
tests are dependent on whether the LLVM build is 64-bit or 32-bit.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D102560