runtime: use correct types for maxstring and concatstring

Updates #6046.
This CL just does maxstring and concatstring. There are other functions
to fix but doing them a few at a time will help isolate any (unlikely)
breakages these changes bring up in architectures I can't test
myself.

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/12519044

src/pkg/runtime/print.c

@@ -350,8 +350,6 @@ runtime·printpointer(void *p)
 void
 runtime·printstring(String v)
 {
-	extern uint32 runtime·maxstring;
-
 	if(v.len > runtime·maxstring) {
 		gwrite("[string too long]", 17);
 		return;

src/pkg/runtime/runtime.h

@@ -706,7 +706,7 @@ extern	int8*	runtime·goos;
 extern	int32	runtime·ncpu;
 extern	bool	runtime·iscgo;
 extern 	void	(*runtime·sysargs)(int32, uint8**);
-extern	uint32	runtime·maxstring;
+extern	uintptr	runtime·maxstring;
 extern	uint32	runtime·Hchansize;
 extern	uint32	runtime·cpuid_ecx;
 extern	uint32	runtime·cpuid_edx;

src/pkg/runtime/string.goc

@@ -35,13 +35,13 @@ runtime·findnullw(uint16 *s)
 	return l;
 }
 
-uint32 runtime·maxstring = 256; // a hint for print
+uintptr runtime·maxstring = 256; // a hint for print
 
 static String
 gostringsize(intgo l)
 {
 	String s;
-	uint32 ms;
+	uintptr ms;
 
 	if(l == 0)
 		return runtime·emptystring;
@@ -51,7 +51,7 @@ gostringsize(intgo l)
 	s.str[l] = 0;
 	for(;;) {
 		ms = runtime·maxstring;
-		if((uint32)l <= ms || runtime·cas(&runtime·maxstring, ms, (uint32)l))
+		if((uintptr)l <= ms || runtime·casp((void**)&runtime·maxstring, (void*)ms, (void*)l))
 			break;
 	}
 	return s;
@@ -176,7 +176,7 @@ concatstring(intgo n, String *s)
 // not have a fixed size argument count.
 #pragma textflag 7
 void
-runtime·concatstring(int32 n, String s1, ...)
+runtime·concatstring(intgo n, String s1, ...)
 {
 	(&s1)[n] = concatstring(n, &s1);
 }