[Hexagon] Fix test on OpenBSD

The test specifies a CPU arch but not a particular OS. So if run on
OpenBSD it acts as if it's an OpenBSD/hexagon system. OpenBSD uses
__guard_local instead of __stack_chk_guard so the test will fail. So
specify an OS other than OpenBSD fixes the test.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D126265