vm_core.h (rb_execution_context_t): interrupt_mask size to match interrupt_flag

rb_atomic_t is 32-bit on 64-bit platforms (including the popular
x86-64 Linux), so save 4 bytes on this structure.  This doesn't
result in any final size reduction due to padding, yet, but
future changes are possible to shrink rb_execution_context_t

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63435 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm_core.h

@@ -787,7 +787,7 @@ typedef struct rb_execution_context_struct {
 
     /* interrupt flags */
     rb_atomic_t interrupt_flag;
-    unsigned long interrupt_mask;
+    rb_atomic_t interrupt_mask; /* size should match flag */
 
     rb_fiber_t *fiber_ptr;
     struct rb_thread_struct *thread_ptr;