Linux: dump the signalled thread first

Linux: dump the signalled thread first

... like the kernel does.

gcore-thread.exp has a check to make sure the signalled thread is the
current thread after loading the core back, but that just works by
accident, because the signalled thread happened to be the last thread
on the thread list, and gdb currently iterates over threads in reverse
order.

So this fixes gcore-thread.exp once we start walking threads in
ascending number.

gdb/ChangeLog:
2015-11-24  Pedro Alves  <palves@redhat.com>

	* linux-tdep.c (find_stop_signal): Delete.
	(struct linux_corefile_thread_data) <pid>: Remove field.
	(linux_corefile_thread_callback): Rename to ...
	(linux_corefile_thread): ... this.  Now takes a struct
	linux_corefile_thread_data pointer rather than a void pointer.
	Remove thread state and thread pid checks.
	(linux_make_corefile_notes): Prefer dumping the signalled thread
	first.  Use ALL_NON_EXITED_THREADS instead of
	iterate_over_threads.