cmd/internal/obj/arm: fix wrong encoding of MULBB

"MULBB R1, R2, R3" is encoded to 0xe163f182, which should be
0xe1630182.

This patch fix it.

fix #20764

Change-Id: I9d3c3ffa40ecde86638e5e083eacc67578caebf4
Reviewed-on: https://go-review.googlesource.com/46491
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -975,7 +975,7 @@ jmp_label_3:
 	MULL	R1, R2, (R4, R3)     // 9231c4e0
 	MULL.S	R1, R2, (R4, R3)     // 9231d4e0
 	MMUL	R1, R2, R3           // 12f153e7
-	MULBB	R1, R2, R3           // 82f163e1
+	MULBB	R1, R2, R3           // 820163e1
 	MULWB	R1, R2, R3           // a20123e1
 	MULWT	R1, R2, R3           // e20123e1
 

src/cmd/internal/obj/arm/asm5.go

@@ -2866,7 +2866,7 @@ func (c *ctxt5) oprrr(p *obj.Prog, a obj.As, sc int) uint32 {
 		return o&(0xf<<28) | 0x12<<20 | 0xa<<4
 
 	case AMULBB:
-		return o&(0xf<<28) | 0x16<<20 | 0xf<<12 | 0x8<<4
+		return o&(0xf<<28) | 0x16<<20 | 0x8<<4
 
 	case AMULAWT:
 		return o&(0xf<<28) | 0x12<<20 | 0xc<<4