cmd/internal/obj/mips: replace MOVD with MOVF on 32-bit to avoid unaligned memory...

cmd/internal/obj/mips: replace MOVD with MOVF on 32-bit to avoid unaligned memory access

This is the simplest CL that I can make for Go 1.8. For Go 1.9, we can revisit it
and optimize the redundant address generation instructions or just fix #599 instead.

Fixes #18140.

Change-Id: Ie4804ab0e00dc6bb318da2bece8035c7c71caac3
Reviewed-on: https://go-review.googlesource.com/34193
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>