Add target method for converting thread handle to thread_info struct pointer

Add target method for converting thread handle to thread_info struct pointer

This patch adds a target method named `to_thread_handle_to_thread_info'.
It is intended to map a thread library specific thread handle (such as
pthread_t for the pthread library) to the corresponding GDB internal
thread_info struct (pointer).

An implementation is provided for Linux pthreads; see linux-thread-db.c.

gdb/ChangeLog:

	* target.h (struct target_ops): Add to_thread_handle_to_thread_info.
	(target_thread_handle_to_thread_info): Declare.
	* target.c (target_thread_handle_to_thread_info): New function.
	* target-delegates.c: Regenerate.
	* gdbthread.h (find_thread_by_handle): Declare.
	* thread.c (find_thread_by_handle): New function.
	* linux-thread-db.c (thread_db_thread_handle_to_thread_info): New
	function.
	(init_thread_db_ops): Register thread_db_thread_handle_to_thread_info.