cmd/internal/obj/loong64: add [X]VFCLASS.{S/D} instructions

cmd/internal/obj/loong64: add [X]VFCLASS.{S/D} instructions

Go asm syntax:
	 VFCLASS{F/D}	VJ, VD
	XVFCLASS{F/D}	XJ, XD

Equivalent platform assembler syntax:
	 vfclass.{s/d}	vd, vj
	xvfclass.{s/d}	xd, xj

Change-Id: Iec373f393be315696d1fefc747a4a5882f993195
Reviewed-on: https://go-review.googlesource.com/c/go/+/670256
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dustin Turner <dustin.turner44@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>