runtime: change nproc local variable to uint32

The nproc and ndone fields are uint32.  This makes the type
consistent.

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/79340044

src/pkg/runtime/mgc0.c

@@ -2033,7 +2033,7 @@ runtime·memorydump(void)
 void
 runtime·gchelper(void)
 {
-	int32 nproc;
+	uint32 nproc;
 
 	gchelperstart();