cmd/internal/obj/mips: standardize on sys.MIPS and sys.MIPS64 constants

cmd/internal/obj/mips: standardize on sys.MIPS and sys.MIPS64 constants

CL 38446 introduced the use of the sys.ArchFamily type into the
cmd/internal/obj/mips package and redefined the mips.Mips32 and
mips.Mips64 constants in terms of their sys.ArchFamily counterparts.
This CL removes these local declarations and consolidates on sys.MIPS
and sys.MIPS64 respectively.

Change-Id: Id7aab6c7fd0de42ff43dde605df6bd4c85a3d895
Reviewed-on: https://go-review.googlesource.com/38287
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>