cmd/internal/obj: remove ppc64 msr support from MOV* insns

These are privileged instructions, and thus will never work with
usermode code. I don't think there is a case where this isn't
true.  The motivation is to simplify handling of MOV* opcodes.

Assembler support for recognizing the MSR as a register is
retained.

Change-Id: Ic33f021a20057b64e69df8ea125e23dd8937e38d
Reviewed-on: https://go-review.googlesource.com/c/go/+/307814


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Carlos Eduardo Seo <carlos.seo@linaro.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Trust: Carlos Eduardo Seo <carlos.seo@linaro.org>