runtime: fix build for arm and ppc64/ppc64le

Change-Id: I17ddcb541dfac8b1e48e01ee005563031b6ade2a
Reviewed-on: https://go-review.googlesource.com/2062Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/noasm.go

@@ -9,6 +9,8 @@
 
 package runtime
 
+import _ "unsafe"
+
 func cmpstring(s1, s2 string) int {
 	l := len(s1)
 	if len(s2) < l {