cmd/asm: add encode tests for MIPS64x

This CL adds basic encode test for mips64x and
most of the instructions are cross checked with 'gas'

Update #35008

Change-Id: I18bb524897aa745bfe23db43fcbb44c3b009463c
Reviewed-on: https://go-review.googlesource.com/c/go/+/204297Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>