cmd/internal/obj/loong64: add {V,XV}{SLL/SRL/SRA/ROTR}[I].{B/H/W/D} instructions...

cmd/internal/obj/loong64: add {V,XV}{SLL/SRL/SRA/ROTR}[I].{B/H/W/D} instructions support

Go asm syntax:
	 V{SLL/SRL/SRA/ROTR}{B/H/W/V}	$1, V2, V3
	XV{SLL/SRL/SRA/ROTR}{B/H/W/V}	$1, X2, X3
	 V{SLL/SRL/SRA/ROTR}{B/H/W/V}	VK, VJ, VD
	XV{SLL/SRL/SRA/ROTR}{B/H/W/V}	XK, XJ, XD

Equivalent platform assembler syntax:
	 v{sll/srl/sra/rotr}i.{b/h/w/d}	v3, v2, $1
	xv{sll/srl/sra/rotr}i.{b/h/w/d}	x3, x2, $1
	 v{sll/srl/sra/rotr}.{b/h/w/d}	vd, vj, vk
	xv{sll/srl/sra/rotr}.{b/h/w/d}	xd, xj, xk

Change-Id: Ie4f04de1c77491a71688d226f7d91cd1a699ab47
Reviewed-on: https://go-review.googlesource.com/c/go/+/637775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>