Fix CodeGen/PowerPC/stack-protector.ll on OpenBSD

On OpenBSD, the stack-smash protection transform uses "__guard_local"
and "__stack_smash_handler" instead of "__stack_chk_guard" and
"__stack_chk_fail".  However, CodeGen/PowerPC/stack-protector.ll
doesn't specify a target OS, so on OpenBSD it fails.

Add -mtriple=ppc32-unknown-linux to make the test host-OS agnostic. While
there, convert to FileCheck.

Patch by Matthew Dempsky.