cmd/internal/obj/loong64: add {,x}vmadd series instructions support

cmd/internal/obj/loong64: add {,x}vmadd series instructions support

Go asm syntax:
	  VMADD{B, H, W, V}                         V1, V2, V3
	  VMSUB{B, H, W, V}                         V1, V2, V3
	 XVMADD{B, H, W, V}                         X1, X2, X3
	 XVMSUB{B, H, W, V}                         X1, X2, X3
	 VMADDWEV{HB, WH, VW,QV}{,U}                V1, V2, V3
	 VMADDWOD{HB, WH, VW,QV}{,U}                V1, V2, V3
	XVMADDWEV{HB, WH, VW,QV}{,U}                X1, X2, X3
	XVMADDWOD{HB, WH, VW,QV}{,U}                X1, X2, X3
	 VMADDWEV{HBUB, WHUH, VWUW, QVUV}           V1, V2, V3
	 VMADDWOD{HBUB, WHUH, VWUW, QVUV}           V1, V2, V3
	XVMADDWEV{HBUB, WHUH, VWUW, QVUV}           X1, X2, X3
	XVMADDWOD{HBUB, WHUH, VWUW, QVUV}           X1, X2, X3

Equivalent platform assembler syntax:
	 vmadd.{b,h,w,d}                            v3, v2, v1
	 vmsub.{b,h,w,d}                            v3, v2, v1
	xvmadd.{b,h,w,d}                            x3, x2, x1
	xvmsub.{b,h,w,d}                            x3, x2, x1
	 vmaddwev.{h.b, w.h, d.w, q.d}{,u}          v3, v2, v1
	 vmaddwod.{h.b, w.h, d.w, q.d}{,u}          v3, v2, v1
	xvmaddwev.{h.b, w.h, d.w, q.d}{,u}          x3, x2, x1
	xvmaddwod.{h.b, w.h, d.w, q.d}{,u}          x3, x2, x1
	 vmaddwev.{h.bu.b, d.wu.w, d.wu.w, q.du.d}  v3, v2, v1
	 vmaddwod.{h.bu.b, d.wu.w, d.wu.w, q.du.d}  v3, v2, v1
	xvmaddwev.{h.bu.b, d.wu.w, d.wu.w, q.du.d}  x3, x2, x1
	xvmaddwod.{h.bu.b, d.wu.w, d.wu.w, q.du.d}  x3, x2, x1

Change-Id: I2f4aae51045e1596d4744e525a1589586065cf8e
Reviewed-on: https://go-review.googlesource.com/c/go/+/724200
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>