[PowerPC] Add linker opt for PC Relative GOT indirect accesses

A linker optimization is available on PowerPC for GOT indirect PCRelative loads.

The idea is that we can mark a usual GOT indirect load:

pld 3, vec@got@pcrel(0), 1
lwa 3, 4(3)

With a relocation to say that if we don't need to go through the GOT we can let
the linker further optimize this and replace a load with a nop.

  pld 3, vec@got@pcrel(0), 1
.Lpcrel1:
.reloc .Lpcrel1-8,R_PPC64_PCREL_OPT,.-(.Lpcrel1-8)
  lwa 3, 4(3)

This patch adds the logic that allows the compiler to add the R_PPC64_PCREL_OPT.

Reviewers: nemanjai, lei, hfinkel, sfertile, efriedma, tstellar, grosbach

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D79864