[AMDGCN] Fix device lib test to work with lib64

This change fixes an issue introduced by
https://reviews.llvm.org/D140315.  A new unit test was added to validate
the search for the device libraries being placed in Clang's resource
directory; however, the test didn't take into account that certain
platforms use lib64 for 64-bit library directory.

Differential Revision: https://reviews.llvm.org/D142506

Change-Id: I9c31a4f08fbc383350d82d6aba01987a3ef63e51