[PowerPC] Fix @got references to local symbols

A @got reference must always result in a relocation, so that
the linker has a chance to set up the GOT entry, even if the
symbol happens to be local.

Add a PPCELFObjectWriter::ExplicitRelSym routine that enforces
a relocation to be emitted for GOT references.
Debug Info: Scope of a DebugLoc should not be null.

No functionality change. Remove handling for the null case.