[lld][PowerPC][test] Avoid flaky failures

This test may fail if there is a new changes to this tests.

The archives are not deleted so the contents from the previous test run
may affect the contents for the current run,
so this will require cleaning up the Output dir or force build of buildbot.

The fix is to put all the objects in the temporary dir that we cleanup every run,
 to avoid run-2-run flaky failures.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D93128