Hide value stored to thread-specific entries for a test purpose

Hide value stored to thread-specific entries for a test purpose

* include/private/specific.h (ts_entry_value_t): New type.
* include/private/specific.h (TS_HIDE_VALUE, TS_REVEAL_PTR): New macro.
* include/private/specific.h (thread_specific_entry): Replace void* to
ts_entry_value_t (for value).
* include/private/specific.h (GC_getspecific): Apply TS_REVEAL_PTR to
entry->value.
* specific.c [USE_CUSTOM_SPECIFIC] (GC_slow_getspecific): Likewise.
* specific.c [USE_CUSTOM_SPECIFIC] (GC_setspecific): Apply TS_HIDE_VALUE
to value stored to entry.