[dev.power64] runtime: Fix broken merge of noasm.go

The earlier dev.power64 merge missed the port of
runtime/noasm.goc to runtime/noasm_arm.go.  This CL fixes this
by moving noasm_arm.go to noasm.go and adding a +build to
share the file between arm and power64.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/158350043

src/runtime/noasm.go

@@ -5,6 +5,8 @@
 // Routines that are implemented in assembly in asm_{amd64,386}.s
 // but are implemented in Go for arm.
 
+// +build arm power64 power64le
+
 package runtime
 
 func cmpstring(s1, s2 string) int {