cmd/internal/obj/loong64: add {,X}V{ADD,SUB}W{EV,OD}.{H.B,W.H,D.W,Q.D}{,U} instructio...

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

Go asm syntax:
	 VADDWEV{HB, WH, VW, QV}{,U}        V1, V2, V3
	 VSUBWEV{HB, WH, VW, QV}{,U}        V1, V2, V3
	 VADDWOD{HB, WH, VW, QV}{,U}        V1, V2, V3
	 VSUBWOD{HB, WH, VW, QV}{,U}        V1, V2, V3
	XVADDWEV{HB, WH, VW, QV}{,U}        X1, X2, X3
	XVSUBWEV{HB, WH, VW, QV}{,U}        X1, X2, X3
	XVADDWOD{HB, WH, VW, QV}{,U}        X1, X2, X3
	XVSUBWOD{HB, WH, VW, QV}{,U}        X1, X2, X3

Equivalent platform assembler syntax:
	 vaddwev.{h.b, w.h, d.w, q.d}{,u}   V3, V2, V1
	 vsubwev.{h.b, w.h, d.w, q.d}{,u}   V3, V2, V1
	 vaddwod.{h.b, w.h, d.w, q.d}{,u}   V3, V2, V1
	 vsubwod.{h.b, w.h, d.w, q.d}{,u}   V3, V2, V1
	xvaddwev.{h.b, w.h, d.w, q.d}{,u}   X3, X2, X1
	xvsubwev.{h.b, w.h, d.w, q.d}{,u}   X3, X2, X1
	xvaddwod.{h.b, w.h, d.w, q.d}{,u}   X3, X2, X1
	xvsubwod.{h.b, w.h, d.w, q.d}{,u}   X3, X2, X1

Change-Id: I407dc65b32b89844fd303e265a99d8aafdf922ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/723620
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>