Fix tests defaulting to incorrect triples on AIX

The tests only specify -march, so when the tests are run on AIX the target OS defaults to AIX, which causes the tests to misbehave.

This patch constrains the tests by specifying -mtriple instead of -march.

Reviewed By: daltenty, jsji, MaskRay

Differential Revision: https://reviews.llvm.org/D110186