Add backlink from lwp_info to thread_info.

Add backlink from lwp_info to thread_info.

	* gdbthread.h (add_thread): Change result type to struct thread_info *.
	* inferiors.c (add_thread): Change result type to struct thread_info *.
	All callers updated.
	(add_lwp): Call add_thread here instead of in callers.
	All callers updated.
	* linux-low.h (get_lwp_thread): Rewrite.
	(struct lwp_info): New member "thread".

This speeds up gdbserver attach in non-stop mode because now get_lwp_thread
doesn't do a linear search for the corresponding thread_info object.