[PowerPC] Add some missing PPC64 relocs

All R_PPC_... relocs should also be present (using the same number)
under the corresponding R_PPC64_... name.   The latter were missing
for a couple of cases, which this patch adds.

This is not a big problem when emitting the reloc, because we can
just use the R_PPC_... define instead.  But it is a problem when
*dumping* relocations e.g. using llvm-readobj, because this will
expect only R_PPC64_... values when inspecting a ppc64 ELF file.
Remove unused isLegalAddressImmediate() method.
[PowerPC] Add ELF relocation tests

This patch extends test/MC/PowerPC/ppc64-fixups.s to not only check for
the correct fixup type in the --show-encoding output, but also runs the
generated object file through llvm-readobj -r and verifies that the
correct ELF relocation records were generated.