MCFixup: Remove FK_PCRel_
The generic FK_Data_ fixup kinds handle both absolute and PC-relative
fixups. ELFObjectWriter sets IsPCRel to true for `.long foo-.`, so the
backend has to handle PC-relative FK_Data_.
However, the existence of FK_PCRel_ encouraged backends to implement it
as a separate fixup type, leading to redundant and error-prone code.
Removing FK_PCRel_ simplifies the overall fixup mechanism.
