Workaround TSan false positives for at-fork handlers accessing lock_holder

Workaround TSan false positives for at-fork handlers accessing lock_holder

* pthread_support.c (GC_wait_for_gc_completion): Skip I_HOLD_LOCK()
assertion if THREAD_SANITIZER and CAN_HANDLE_FORK; add comment.
* pthread_support.c [CAN_HANDLE_FORK && GC_ASSERTIONS]
(fork_prepare_proc, fork_parent_proc, fork_child_proc): Add
GC_ATTR_NO_SANITIZE_THREAD attribute.