cmd/internal/obj/loong64: Add preld{,x} instructions support

cmd/internal/obj/loong64: Add preld{,x} instructions support

Go asm syntax:
	PRELD	16(R4), $8
	PRELDX	(R4)(R5), $8

Equivalent platform assembler syntax:
	preld	$8, $r4, 16
	preldx	$8, $r4, $r5

Change-Id: Ie81d22ebaf4153388a7e9d8fa0f618a0ae7a1c9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/671875
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>