math: ARM assembly implementation for Abs

Obtained on 700MHz OMAP4460:
benchmark       old ns/op    new ns/op    delta
BenchmarkAbs           61           23  -61.63%

R=dave, remyoudompheng, mtj, rsc
CC=golang-dev
https://golang.org/cl/6094047

src/pkg/math/abs_arm.s

@@ -3,4 +3,9 @@
 // license that can be found in the LICENSE file.
 
 TEXT ·Abs(SB),7,$0
-	B ·abs(SB)
+	MOVW	lo+0(FP), R0
+	MOVW	hi+4(FP), R1
+	AND 	$((1<<31)-1), R1
+	MOVW	R0, resultlo+8(FP)
+	MOVW	R1, resulthi+12(FP)
+	RET