Fix a retainer profiling segfault

The bitmap type wasn't big enough to hold large bitmaps on 64 bit
platforms. Profiling GHC was segfaulting when retainStack was handling a
size 33 bitmap.

rts/RetainerProfile.c

@@ -1270,7 +1270,7 @@ retainStack( StgClosure *c, retainer c_child_r,
     stackElement *oldStackBoundary;
     StgPtr p;
     StgRetInfoTable *info;
-    StgWord32 bitmap;
+    StgWord bitmap;
     nat size;
 
 #ifdef DEBUG_RETAINER