cmd/asm: add MIPS MSA LD/ST/LDI support for mips64x

This CL adding primitive asm support of MIPS MSA by introducing
new sets of register W0-W31 (C_WREG) and 12 new instructions:

* VMOV{B,H,W,D} ADDCONST, WREG  (Vector load immediate)
* VMOV{B,H,W,D} SOREG, WREG     (Vector load)
* VMOV{B,H,W,D} WREG, SOREG     (Vector store)

Ref: MIPS Architecture for Programmers Volume IV-j: The MIPS64 SIMD Architecture Module

Change-Id: I3362c59a73c82c94769c18a19a0bee7e5029217d
Reviewed-on: https://go-review.googlesource.com/c/go/+/215723


Run-TryBot: Meng Zhuo <mengzhuo1203@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>