[LLD] Use Rela on PowerPC too

Patch by Nicholas Allegra.

The spec for ELF on PowerPC:
http://refspecs.linux-foundation.org/elf/elfspec_ppc.pdf
says:
"The PowerPC family uses only the Elf32_Rela relocation entries with
explicit addends."

(EM_PPC64 should be covered by Config->Is64 already.)

Differential Revision: https://reviews.llvm.org/D40826

llvm-svn: 323843