runtime: use GOTRACEBACK to decide whether to show runtime frames

Right now, GOTRACEBACK=0 means do not show any stack traces.
Unset means the default behavior (declutter by hiding runtime routines).

This CL makes GOTRACEBACK=2 mean include the runtime routines.
It avoids having to recompile the runtime when you want to see
the runtime in the tracebacks.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5633050

src/pkg/runtime/symtab.c

@@ -507,6 +507,9 @@ contains(String s, int8 *p)
 bool
 runtime·showframe(Func *f)
 {
-	// return 1;  // for debugging - show all frames
-	return contains(f->name, ".") && !hasprefix(f->name, "runtime.");
+	static int32 traceback = -1;
+	
+	if(traceback < 0)
+		traceback = runtime·gotraceback();
+	return traceback > 1 || contains(f->name, ".") && !hasprefix(f->name, "runtime.");
 }