cmd/internal/obj: reorder ppc64 MOV* optab entries

These are always sorted and grouped during initialization of the
actual opcode -> optab map generation. Thus, their initial location
in optab is mostly aimed at readability. This cleanup is intends to
ease reviewing of future patches which simplify, combine, or remove
MOV* optab entries.

Change-Id: I87583ed34fab79e0f625880f419d499939e2a9e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/300612
Reviewed-by: Lynn Boger <[email protected]>
Trust: Emmanuel Odeke <[email protected]>

Author: pmur