Make `trace_running` an integer flag again

* vm_core.h (rb_vm_struct): trace_running should be a counter but
  not a bit flag.  [ruby-core:78514] [Bug #13011]

Author: David Rodríguez <deivid.rodriguez@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57002 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -497,7 +497,7 @@ typedef struct rb_vm_struct {
     unsigned int running: 1;
     unsigned int thread_abort_on_exception: 1;
     unsigned int thread_report_on_exception: 1;
-    unsigned int trace_running: 1;
+    int trace_running;
     volatile int sleeper;
 
     /* object management */