Fix checks of thread-local free-lists.

Fix checks of thread-local free-lists.

* include/private/gc_priv.h, alloc.c (GC_check_fl_marks): Re-read each
pointer before following the pointed-to link, and bail out if the result is
different.  This can happen if the thread has popped the object off the
free-list.  The prototype is changed to allow re-reading the pointers.
* thread_local_alloc.c (GC_check_tls_for): Adjust the calls accordingly.