cmd/internal/obj/mips: use LR as temporary register in RET

cmd/internal/obj/mips: use LR as temporary register in RET

On mips/mips64, for non-leaf function, RET is assembled as
	MOV (SP), R4       // load saved LR
	ADD $framesize, SP
	JMP (R4)

This clobbers R4 unnecessarily. Use the link register as
temporary instead.

Probably for Go 1.11.

Change-Id: I2209db7be11074ed2e0e0829cace95ebfb709e9f
Reviewed-on: https://go-review.googlesource.com/79016
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Austin Clements <austin@google.com>