runtime: use enums instead static vars for debugging

Compiler can detect and delete dead code with enums,
but can not with static vars.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/9377043

src/pkg/runtime/chan.c

@@ -11,8 +11,6 @@
 #define	MAXALIGN	7
 #define	NOSELGEN	1
 
-static	int32	debug	= 0;
-
 typedef	struct	WaitQ	WaitQ;
 typedef	struct	SudoG	SudoG;
 typedef	struct	Select	Select;
@@ -58,6 +56,8 @@ uint32 runtime·Hchansize = sizeof(Hchan);
 
 enum
 {
+	debug = 0,
+
 	// Scase.kind
 	CaseRecv,
 	CaseSend,

src/pkg/runtime/slice.c

@@ -9,7 +9,10 @@
 #include "malloc.h"
 #include "race.h"
 
-static	bool	debug	= 0;
+enum
+{
+	debug = 0
+};
 
 static	void	makeslice1(SliceType*, intgo, intgo, Slice*);
 static	void	growslice1(SliceType*, Slice, intgo, Slice *);