bytes: add assembly version of IndexByte for ARM

benchmark                        old ns/op    new ns/op    delta
BenchmarkIndexByte32                   459          126  -72.55%
BenchmarkIndexByte4K                 52404        10939  -79.13%
BenchmarkIndexByte4M              54470800     11177370  -79.48%
BenchmarkIndexByte64M           1010803000    178860500  -82.31%

benchmark                         old MB/s     new MB/s  speedup
BenchmarkIndexByte32                 69.58       252.63    3.63x
BenchmarkIndexByte4K                 78.16       374.42    4.79x
BenchmarkIndexByte4M                 77.00       375.25    4.87x
BenchmarkIndexByte64M                66.39       375.20    5.65x

R=rsc, minux.ma
CC=golang-dev
https://golang.org/cl/6106044

src/pkg/bytes/asm_arm.s

@@ -2,10 +2,29 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// no memchr implementation on arm yet
 TEXT ·IndexByte(SB),7,$0
-	B	·indexBytePortable(SB)
+	MOVW	base+0(FP), R0
+	MOVW	len+4(FP), R1
+	MOVBU	c+12(FP), R2	// byte to find
+	MOVW	R0, R4		// store base for later
+	ADD	R0, R1		// end 
+
+_loop:
+	CMP	R0, R1
+	B.EQ	_notfound
+	MOVBU.P	1(R0), R3
+	CMP	R2, R3
+	B.NE	_loop
+
+	SUB	$1, R0		// R0 will be one beyond the position we want
+	SUB	R4, R0		// remove base
+	MOVW    R0, index+16(FP) 
+	RET
+
+_notfound:
+	MOVW	$-1, R0
+	MOVW	R0, index+16(FP)
+	RET
 
-// no memcmp implementation on arm yet
 TEXT ·Equal(SB),7,$0
 	B	·equalPortable(SB)