cmd/internal/obj/loong64: add {,X}VS{ADD,SUB}.{B/H/W/V}{,U} instructions support

cmd/internal/obj/loong64: add {,X}VS{ADD,SUB}.{B/H/W/V}{,U} instructions support

Go asm syntax:
	 VSADD{B,H,W,V}     V1, V2, V3
	 VSADD{B,H,W,V}U    V1, V2, V3
	 VSSUB{B,H,W,V}     V1, V2, V3
	 VSSUB{B,H,W,V}U    V1, V2, V3
	XVSADD{B,H,W,V}     X1, X2, X3
	XVSADD{B,H,W,V}U    X1, X2, X3
	XVSSUB{B,H,W,V}     X1, X2, X3
	XVSSUB{B,H,W,V}U    X1, X2, X3

Equivalent platform assembler syntax:
	 vsadd.{b,h,w,d}    v3, v2, v1
	 vsadd.{b,h,w,d}u   v3, v2, v1
	 vssub.{b,h,w,d}    v3, v2, v1
	 vssub.{b,h,w,d}u   v3, v2, v1
	xvsadd.{b,h,w,d}    x3, x2, x1
	xvsadd.{b,h,w,d}u   x3, x2, x1
	xvssub.{b,h,w,d}    x3, x2, x1
	xvssub.{b,h,w,d}u   x3, x2, x1

Change-Id: Iab8c1a9bdc4940598936f3beac846466e913ffa2
Reviewed-on: https://go-review.googlesource.com/c/go/+/719200
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Michael Pratt <mpratt@google.com>