Use union of AO_t and 'tse' data pointer in specific.c to avoid type

Use union of AO_t and 'tse' data pointer in specific.c to avoid type
casting (breaking strict-aliasing rules)

* include/private/specific.h (ptse_ao_u): New union type.
* include/private/specific.h (thread_specific_data): Change type of
"hash" field from "tse" pointer to ptse_ao_u.
* specific.c (invalid_tse): Put to 'const' data section.
* specific.c (GC_key_create_inner, GC_setspecific, GC_remove_specific,
GC_slow_getspecific, GC_check_tsd_marks): Use "p" element of union to
access "tse" pointer of hash table.
* specific.c (GC_setspecific): Use "ao" element of union to store
"hash" table entry atomically; remove type cast.