cmd/internal/obj/loong64: add support for instructions ANDN and ORN

Go asm syntax:
	ANDN/ORN	RK, RJ, RD
    or  ANDN/ORN	RK, RD

Equivalent platform assembler syntax:
	andn/orn	rd, rj, rk
    or  andn/orn	rd, rd, rk

Ref: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html

Change-Id: I6d240ecae8f9443811ca450aed3574f13f0f4a81
Reviewed-on: https://go-review.googlesource.com/c/go/+/610475
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Commit-Queue: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: abner chenc <chenguoqi@loongson.cn>

src/cmd/asm/internal/asm/testdata/loong64enc1.s

@@ -132,6 +132,11 @@ lable2:
 	BREAK				// 00002a00
 	UNDEF				// 00002a00
 
+	ANDN	R4, R5, R6		// a6901600
+	ANDN	R4, R5			// a5901600
+	ORN	R4, R5, R6		// a6101600
+	ORN	R4, R5			// a5101600
+
 	// mul
 	MUL	R4, R5	  		// a5101c00
 	MUL	R4, R5, R6	  	// a6101c00

src/cmd/internal/obj/loong64/a.out.go

@@ -385,6 +385,10 @@ const (
 	AMOVVF
 	AMOVVD
 
+	// 2.2.1.8
+	AORN
+	AANDN
+
 	// 2.2.7. Atomic Memory Access Instructions
 	AAMSWAPB
 	AAMSWAPH

src/cmd/internal/obj/loong64/anames.go

@@ -127,6 +127,8 @@ var Anames = []string{
 	"MOVDV",
 	"MOVVF",
 	"MOVVD",
+	"ORN",
+	"ANDN",
 	"AMSWAPB",
 	"AMSWAPH",
 	"AMSWAPW",

src/cmd/internal/obj/loong64/asm.go

@@ -1100,6 +1100,8 @@ func buildop(ctxt *obj.Link) {
 		case AAND:
 			opset(AOR, r0)
 			opset(AXOR, r0)
+			opset(AORN, r0)
+			opset(AANDN, r0)
 
 		case ABEQ:
 			opset(ABNE, r0)
@@ -1858,6 +1860,10 @@ func (c *ctxt0) oprrr(a obj.As) uint32 {
 		return 0x2a << 15
 	case AXOR:
 		return 0x2b << 15
+	case AORN:
+		return 0x2c << 15 // orn
+	case AANDN:
+		return 0x2d << 15 // andn
 	case ASUB:
 		return 0x22 << 15
 	case ASUBU, ANEGW: