MCFixup: Remove FK_PCRel_ from getKindForSize

Remove FK_PCRel_* kinds from the generic fixup list, as they are not
generic like FK_Data_*. In getRelocType, FK_PCRel_* can be replaced with
FK_Data_* by leveraging the IsPCRel argument. Their inclusion in the
generic kind list caused confusion for PowerPC, RISCV, and VE targets.

The X86/M68k uses can be implemented as target-specific fixups.