cmd/internal/obj/loong64: add LDPTR.{W/D} and STPTR.{W/D} instructions support

cmd/internal/obj/loong64: add LDPTR.{W/D} and STPTR.{W/D} instructions support

Go asm syntax:
	MOVWP		4(R4), R5
	MOVVP		8(R4), R5
	MOVWP		R4, 12(R5)
	MOVVP		R4, 16(R5)

Equivalent platform assembler syntax:
	ldptr.w		r5, r4, $1
	ldptr.d		r5, r4, $2
	stptr.w		r4, r5, $3
	stptr.d		r4, r5, $4

Change-Id: I50a341cee2d875cb7c5da9db08b23799c9dc6c64
Reviewed-on: https://go-review.googlesource.com/c/go/+/699055
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>