cmd/internal/obj/loong64: add {,X}VSLT.{B/H/W/V}{,U} instructions support

cmd/internal/obj/loong64: add {,X}VSLT.{B/H/W/V}{,U} instructions support

  Go asm syntax:
	 VSLT{B,H,W,V}    V1,   V2, V3
	 VSLT{B,H,W,V}U   V1,   V2, V3
	 VSLT{B,H,W,V}    $-2,  V2, V3
	 VSLT{B,H,W,V}U   $8,   V2, V3
	XVSLT{B,H,W,V}    X1,   X2, X3
	XVSLT{B,H,W,V}U   X1,   X2, X3
	XVSLT{B,H,W,V}    $-16, X2, X3
	XVSLT{B,H,W,V}U   $31,  X2, X3

  Equivalent platform assembler syntax:
	 vslt.{b,h,w,d}    v3, v2, v1
	 vslt.{b,h,w,d}u   v3, v2, v1
	 vslti.{b,h,w,d}   v3, v2, $-2
	 vslti.{b,h,w,d}u  v3, v2, $8
	xvslt.{b,h,w,d}    x3, x2, x1
	xvslt.{b,h,w,d}u   x3, x2, x1
	xvslti.{b,h,w,d}   x3, x2, $-16
	xvslti.{b,h,w,d}u  x3, x2, $31

Change-Id: Iccfb65c0c19b62d2c5ec279a077393c68e1bf7d4
Reviewed-on: https://go-review.googlesource.com/c/go/+/721620
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: sophie zhao <zhaoxiaolin@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>