bytes: add assembly version of Equal for ARM

BenchmarkEqual32                       662          159  -75.98%
BenchmarkEqual4K                     76545        13719  -82.08%
BenchmarkEqual4M                  90136700     23588870  -73.83%
BenchmarkEqual64M               2147483647   1419616000  -42.63%

BenchmarkEqual32                     48.32       201.15    4.16x
BenchmarkEqual4K                     53.51       298.56    5.58x
BenchmarkEqual4M                     46.53       177.81    3.82x
BenchmarkEqual64M                    27.12        47.27    1.74x

R=golang-dev, qyzhai, minux.ma, rsc, iant, nigeltao
CC=golang-dev
https://golang.org/cl/6118049

src/pkg/bytes/asm_arm.s

@@ -27,4 +27,30 @@ _notfound:
 	RET
 
 TEXT ·Equal(SB),7,$0
-	B	·equalPortable(SB)
+	MOVW	alen+4(FP), R1
+	MOVW	blen+16(FP), R3
+	
+	CMP	R1, R3		// unequal lengths are not equal
+	B.NE	_notequal
+
+	MOVW	aptr+0(FP), R0
+	MOVW	bptr+12(FP), R2
+	ADD	R0, R1		// end
+
+_next:
+	CMP	R0, R1
+	B.EQ	_equal		// reached the end
+	MOVBU.P	1(R0), R4
+	MOVBU.P	1(R2), R5
+	CMP	R4, R5
+	B.EQ	_next
+
+_notequal:
+	MOVW	$0, R0
+	MOVW	R0, equal+24(FP)
+	RET
+
+_equal:
+	MOVW	$1, R0
+	MOVW	R0, equal+24(FP)
+	RET