runtime: fix data race on runtime·maxstring

The data race can lead to erroneous output of
"[invalid string]" instead of a string.

R=golang-dev
CC=golang-dev
https://golang.org/cl/4678049

src/pkg/runtime/print.c

@@ -320,7 +320,7 @@ runtime·printpointer(void *p)
 void
 runtime·printstring(String v)
 {
-	extern int32 runtime·maxstring;
+	extern uint32 runtime·maxstring;
 
 	if(v.len > runtime·maxstring) {
 		runtime·write(2, "[invalid string]", 16);

src/pkg/runtime/string.goc

@@ -32,19 +32,23 @@ runtime·findnullw(uint16 *s)
 	return l;
 }
 
-int32 runtime·maxstring = 256;
+uint32 runtime·maxstring = 256;
 
 String
 runtime·gostringsize(int32 l)
 {
 	String s;
+	uint32 ms;
 
 	if(l == 0)
 		return runtime·emptystring;
 	s.str = runtime·mal(l+1);	// leave room for NUL for C runtime (e.g., callers of getenv)
 	s.len = l;
-	if(l > runtime·maxstring)
-		runtime·maxstring = l;
+	for(;;) {
+		ms = runtime·maxstring;
+		if((uint32)l <= ms || runtime·cas(&runtime·maxstring, ms, (uint32)l))
+			break;
+	}
 	return s;
 }