cmd/internal/obj/mips: store LR before update SP in function prologue

cmd/internal/obj/mips: store LR before update SP in function prologue

This prevents the traceback code from seeing a half-updated
stack frame when a profiling signal comes during the execution
of function prologue. Also fixes mips64x part of #17381.

Change-Id: Iec9683427e546e3648b2e8b1dde956d13f6eb938
Reviewed-on: https://go-review.googlesource.com/31721
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>