cmd/internal/obj/loong64: add [X]VF{ADD/SUB/MUL/DIV}.{S/D} instructions

Go asm syntax:
	 V{ADD/SUB/MUL/DIV}{F/D}	VK, VJ, VD
	XV{ADD/SUB/MUL/DIV}{F/D}	XK, XJ, XD

Equivalent platform assembler syntax:
	 vf{add/sub/mul/div}.{s/d}	vd, vj, vk
	xvf{add/sub/mul/div}.{s/d}	xd, xj, xk

Change-Id: I4607884212167ac97d7b6448ea3c849fc0fdd506
Reviewed-on: https://go-review.googlesource.com/c/go/+/670255


Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>