runtime: avoid defining the same variable in more than one translation unit

For gccgo runtime and Darwin where -fno-common is the default.

R=iant, dave
CC=golang-dev
https://golang.org/cl/7094061

src/pkg/runtime/malloc.goc

@@ -19,6 +19,8 @@ package runtime
 #pragma dataflag 16 /* mark mheap as 'no pointers', hiding from garbage collector */
 MHeap runtime·mheap;
 
+int32	runtime·checking;
+
 extern MStats mstats;	// defined in zruntime_def_$GOOS_$GOARCH.go
 
 extern volatile intgo runtime·MemProfileRate;

src/pkg/runtime/malloc.h

@@ -446,7 +446,7 @@ void	runtime·markallocated(void *v, uintptr n, bool noptr);
 void	runtime·checkallocated(void *v, uintptr n);
 void	runtime·markfreed(void *v, uintptr n);
 void	runtime·checkfreed(void *v, uintptr n);
-int32	runtime·checking;
+extern	int32	runtime·checking;
 void	runtime·markspan(void *v, uintptr size, uintptr n, bool leftover);
 void	runtime·unmarkspan(void *v, uintptr size);
 bool	runtime·blockspecial(void*);

src/pkg/runtime/proc.c

@@ -24,6 +24,13 @@ static	int32	debug	= 0;
 
 int32	runtime·gcwaiting;
 
+G*	runtime·allg;
+G*	runtime·lastg;
+M*	runtime·allm;
+
+int8*	runtime·goos;
+int32	runtime·ncpu;
+
 // Go scheduler
 //
 // The go scheduler's job is to match ready-to-run goroutines (`g's)

src/pkg/runtime/runtime.h

@@ -562,15 +562,15 @@ struct Panic
  */
 extern	String	runtime·emptystring;
 extern	uintptr runtime·zerobase;
-G*	runtime·allg;
-G*	runtime·lastg;
-M*	runtime·allm;
+extern	G*	runtime·allg;
+extern	G*	runtime·lastg;
+extern	M*	runtime·allm;
 extern	int32	runtime·gomaxprocs;
 extern	bool	runtime·singleproc;
 extern	uint32	runtime·panicking;
 extern	int32	runtime·gcwaiting;		// gc is waiting to run
-int8*	runtime·goos;
-int32	runtime·ncpu;
+extern	int8*	runtime·goos;
+extern	int32	runtime·ncpu;
 extern	bool	runtime·iscgo;
 extern 	void	(*runtime·sysargs)(int32, uint8**);
 extern	uint32	runtime·maxstring;