cmd/internal/obj/loong64: add [X]VFRINT[{RNE/RZ/RP/RM}].{S/D} instructions

cmd/internal/obj/loong64: add [X]VFRINT[{RNE/RZ/RP/RM}].{S/D} instructions

Go asm syntax:
	 VFRINT[{RNE/RZ/RP/RM}]{F/D}	VJ, VD
	XVFRINT[{RNE/RZ/RP/RM}]{F/D}	XJ, XD

Equivalent platform assembler syntax:
	 vfrint[{rne/rz/rp/rm}].{s/d}	vd, vj
	xvfrint[{rne/rz/rp/rm}].{s/d}	xd, xj

Change-Id: I4ed8782289ae3329d675239f799d5f75b1adc4ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/670235
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>