DMD 0.68, on RedHat 9

When the GC runs twice, the second run segfaults.  It turns out that it segfaults when the code attempts to use the Thread reference.  For some reason, the Thread object was cleaned up on the first run of the GC!

The following code generates an error:

	import gc;
	import c.stdio;
 	int main(char[][] args)
	{
	  printf("Starting test!\n"); fflush(null);
	  gc.fullCollect();
	  gc.fullCollect();
	  return 0;
	};

The output, after I added some trace statements into phobos, looks like this:

	fullcollect: START
	fullcollect: END
	Starting test!
	fullcollect: START
	 threads:0/1 t=0x401b9fc0 *(long*)t=0x804e129Thread.~this(): this=0x401b9fc0
	fullcollect: END
	fullcollect: START
	 threads:0/1 t=0x401b9fc0 *(long*)t=(nil)Segmentation fault

The key thing to note is that Thread.~this() gets called during fullcollect().