Disable tracking of big values in the Julia GC by default.

This suppresses a potential problem with marking large objects as
roots.

src/julia_gc.c

@@ -40,7 +40,7 @@
 // if DISABLE_BIGVAL_TRACKING is defined, we don't track the location of
 // large bags; this speeds up some things, but may expose bugs in GAP code
 // which incorrectly holds pointers into bags over a GC.
-// #define DISABLE_BIGVAL_TRACKING
+#define DISABLE_BIGVAL_TRACKING
 
 // if REQUIRE_PRECISE_MARKING is defined, we assume that all marking
 // functions are precise, i.e., they only invoke MarkBag on valid bags,