[ELF][PPC] Fix getRelExpr for R_PPC64_REL16_HI

Fixes https://github.com/ClangBuiltLinux/linux/issues/640

R_PPC64_REL16_HI was incorrectly computed as an R_ABS relocation.
rLLD368964 made it a linker failure. Change it to use R_PC to fix the
failures.

Add ppc64-reloc-rel.s for these R_PPC64_REL* tests.

llvm-svn: 369184