[AIX][Test][ORC] Skip unsupported ORC C API tests on AIX

As mentioned before in D78813, 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 disables the tests on AIX for now.
This is consistent with what's been done, for example, regarding
`armv7`.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D101971