Revert "r250372 - [ELF2/PPC64] Remove now-dead weak/undef check"

Unfortunately, the check was not as dead as I had thought, and adjusting the
starting VA again exposed the problem. We end up trying to relocate the bl
(using a 24-bit relative offset) to a symbol address of zero, and in general,
that does not fit.

Thus, reverting for now, and adding a test case.

llvm-svn: 250423