cmd/asm: add rotr/drotr for mips64

This CL encodes:

ROTR rd, rt, sa
ROTRV rd, rt, rs

=> ROTR (SCON|REG), (REG,)? REG

DROTR rd, rt, sa
DROTR32 rd, rt, sa
DROTRV rd, rt, rs

=> ROTRV (SCON|REG), (REG,)? REG

Note: ROTRV will handle const over 32
Ref: The MIPS64® Instruction Set Reference Manual Revision 6.05
Change-Id: Ibe69f999b83eb43843d088cf1ac5a13c995269a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/280114


Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>