cmd/internal/obj/mips: copy cmd/internal/obj/ppc64

Just a mechanical copy with filename renames, no code changes.
This is to reduce code difference when adding the mips64 port.

Change-Id: Id06e975f414a7b09f4827167b30813b228a3bfae
Reviewed-on: https://go-review.googlesource.com/14323
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: minux