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>

Author: davecheney

src/cmd/internal/obj/mips/a.out.go

@@ -31,7 +31,6 @@ package mips
 
 import (
 	"cmd/internal/obj"
-	"cmd/internal/sys"
 )
 
 //go:generate go run ../stringer.go -i $GOFILE -o anames.go -p mips
@@ -220,11 +219,6 @@ const (
 	NSCHED = 20
 )
 
-const (
-	Mips32 = sys.MIPS
-	Mips64 = sys.MIPS64
-)
-
 const (
 	C_NONE = iota
 	C_REG