Fix 'bogus LR' detection in FindTopOfStack (Darwin)

Fix 'bogus LR' detection in FindTopOfStack (Darwin)
(fix commit 5742f86)

* darwin_stop_world.c [!DARWIN_DONT_PARSE_STACK] (GC_FindTopOfStack):
Use "UL" suffix (instead of "U") for 0x3 (on the right side of the
comparison) to avoid implicit unsigned int-to-long value extension
of "~" operator result.