[ELF][PPC] Refactor some ppc64 tests

Merge ppc64-dynamic-relocations.s into ppc64-plt-stub.s
Add ppc64-tls-ie.s: covers ppc64-initial-exec-tls.s and ppc64-tls-ie-le.s
Add ppc64-tls-gd.s: covers ppc64-general-dynamic-tls.s, ppc64-gd-to-ie.s, ppc64-tls-gd-le.s, and ppc64-tls-gd-le-small.s

llvm-svn: 366424