Do not add no_sanitize_thread to fork at-handlers if no pthread_atfork

Do not add no_sanitize_thread to fork at-handlers if no pthread_atfork
(fix commit 3a52469)

* pthread_support.c [THREAD_SANITIZER] (GC_wait_for_gc_completion): Do
not skip I_HOLD_LOCK() assertion unless CAN_CALL_ATFORK.
* pthread_support.c [CAN_HANDLE_FORK && GC_ASSERTIONS]
(fork_prepare_proc, fork_parent_proc, fork_child_proc): Do not add
GC_ATTR_NO_SANITIZE_THREAD attribute unless CAN_CALL_ATFORK.