cmd/internal/obj/loong64: add new instructions ALSL{W/WU/V} for loong64

cmd/internal/obj/loong64: add new instructions ALSL{W/WU/V} for loong64

Go asm syntax:
	ALSL{W/WU/V}	$3, R4, R5, R6

Equivalent platform assembler syntax:
	alsl.{w/wu/d}	$r6, $r4, $r5, 3

Change-Id: Ic8364dfe2753bcea7de6cffe656ca0dde6875766
Reviewed-on: https://go-review.googlesource.com/c/go/+/692136
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>