[PowerPC] Make test case buildable with GNU as

The ppc64-fixups.s test currently fails to build with GNU as, since it
does not support plain symbols as arguments to li/lis.  Rewrite the test
for R_PPC64_ADDR16 and R_PPC64_REL16 to use lwz instead.

Allowing the test case to be built with both LLVM and GNU as makes it
easier to spot unwanted difference in the output.