[ELF][PPC64] Fix assertion failure for branches to hidden undefined weak for -no-pie

Reported by Stefan Pintilie in D119773.

For a branch to a hidden undefined weak symbol, there is an
`assert(sym->getVA());` failure in PPC64LongBranchTargetSection::writeTo for a
-no-pie link. The root cause is that we unnecessarily create the thunk for the
-no-pie link.

Fix this by changing the condition to just `s.isUndefined()`. See the inline
comment.

Rename ppc64-weak-undef-call.s to ppc64-undefined-weak.s to be consistent with
other architectures.

Reviewed By: sfertile, stefanp

Differential Revision: https://reviews.llvm.org/D119787