Fix removal of dead threads in a child process

Fix removal of dead threads in a child process

GC_threads table may contain several elements with the same pthread id,
so GC_remove_all_threads_but_me should remove all elements except for
the first found element matching the id of the current thread.

* pthread_support.c [CAN_HANDLE_FORK] (GC_remove_all_threads_but_me):
Do not assign me if it is already non-null; add comment.
* win32_threads.c [CAN_HANDLE_FORK] (GC_remove_all_threads_but_me):
Likewise.
* win32_threads.c [CAN_HANDLE_FORK] (GC_remove_all_threads_but_me):
Remove GC_ASSERT(me==NULL).