cmd/internal/obj: make ppc64's CR subregisters print as CRn rather than Cn

cmd/internal/obj: make ppc64's CR subregisters print as CRn rather than Cn

These 8 registers are windows into the CR register. They are officially CR0
through CR7 and that is what the assembler accepts, but for some reason
they have always printed as C0 through C7. Fix the naming and printing.

Change-Id: I55822c0322c29d3e01a1f2776b3b210ebf9ded21
Reviewed-on: https://go-review.googlesource.com/6290
Reviewed-by: Russ Cox <rsc@golang.org>