cmd/internal/obj/loong64: add {V,XV}{ADD/SUB}.{B,H,W,D,Q} and {V,XV}{ADD/SUB}.{B...

cmd/internal/obj/loong64: add {V,XV}{ADD/SUB}.{B,H,W,D,Q} and {V,XV}{ADD/SUB}.{B,H,W,D}Uinstructions support

Go asm syntax:
	 V{ADD/SUB}{B,H,W,V,Q}		VK, VJ, VD
	XV{ADD/SUB}{B,H,W,V,Q}		XK, XJ, XD
	 V{ADD/SUB}{B,H,W,V}U		$1, VJ, VD
	XV{ADD/SUB}{B,H,W,V}U		$1, XJ, XD

Equivalent platform assembler syntax:
	 v{add/sub}.{b,h,w,d,q}		vd, vj, vk
	xv{add/sub}.{b,h,w,d,q}		xd, xj, xk
	 v{add/sub}i.{b,h,w,d}u		vd, vj, $1
	xv{add/sub}i.{b,h,w,d}u		xd, xj, $1

Change-Id: Ia1ef0bc062f4403bb0b1514c2cf1c0264f5d22ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/637795
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>