Add PowerPC 64-bit DWARF register mappings

[tmcgilchrist]

Using the definitions from https://openpowerfoundation.org/specifications/64bitelfabi/ with a small test to check the register mappings. I followed the pattern from #574 .

Are there other tests required for these changes? I am testing using my own Rust project that inspects ELF binaries and extracts debug information from them.

The Power32 bindings should be quite similar according to http://refspecs.linux-foundation.org/elf/elfspec_ppc.pdf and https://github.com/torvalds/linux/blob/master/arch/powerpc/perf/perf_regs.c

[philipc]

That's fine for the testing. The only better test we could do is check the dwarfdump output on a powerpc binary, but we don't do that sort of thing currently, so as long as you've tested that manually it's fine.

[philipc]

Thanks!