Add assertion ensuring proper alignment of 'pushed' GC symbols

Add assertion ensuring proper alignment of 'pushed' GC symbols

* finalize.c (GC_push_finalizer_structures): Assert word alignment of
dl_head, fo_head, GC_finalize_now.
* pthread_support.c (GC_thr_init): Assert word alignment of GC_threads
(passed to GC_push_all).
* win32_threads.c (GC_thr_init): Likewise.
* thread_local_alloc.c (GC_init_thread_local): Assert word alignment
of GC_thread_key.
* typd_mlc.c (GC_add_ext_descriptor): Assert word alignment of
GC_ext_descriptors.