cmd/internal/obj/loong64: add {V,XV}{BITCLR/BITSET/BITREV}[I].{B/H/W/D} instructions...

cmd/internal/obj/loong64: add {V,XV}{BITCLR/BITSET/BITREV}[I].{B/H/W/D} instructions support

Go asm syntax:
	 V{BITCLR/BITSET/BITREV}{B/H/W/V}	$1, V2, V3
	XV{BITCLR/BITSET/BITREV}{B/H/W/V}	$1, X2, X3
	 V{BITCLR/BITSET/BITREV}{B/H/W/V}	VK, VJ, VD
	XV{BITCLR/BITSET/BITREV}{B/H/W/V}	XK, XJ, XD

Equivalent platform assembler syntax:
	 v{bitclr/bitset/bitrev}i.{b/h/w/d}	v3, v2, $1
	xv{bitclr/bitset/bitrev}i.{b/h/w/d}	x3, x2, $1
	 v{bitclr/bitset/bitrev}.{b/h/w/d}	vd, vj, vk
	xv{bitclr/bitset/bitrev}.{b/h/w/d}	xd, xj, xk

Change-Id: I244f8ae316f72cc7ea01ca0139ac78c5616a3c5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/677435
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Mark Freeman <mark@golang.org>