[ELF][PPC64] Fix getRelExpr for R_PPC64_REL16_LO and R_PPC64_REL16_HA

The relocations R_PPC64_REL16_LO and R_PPC64_REL16_HA should return R_PC
for getRelExpr since they compute #lo(S + A – P) and #ha(S + A – P).

Differential Revision: https://reviews.llvm.org/D44648

llvm-svn: 328103