[PowerPC][Future] Add offsets to PC Relative relocations.

This is an optimization that applies to global addresses and
allows for the following transformation:
Convert this:

paddi r3, 0, symbol@PCREL, 1
ld r4, 8(r3)

To this:

pld r4, symbol@PCREL+8(0), 1

An instruction is saved and the linker can do the addition when
the symbol is resolved.

Differential Revision: https://reviews.llvm.org/D76160