Add constructor and destructor to thread_info

Add constructor and destructor to thread_info

This patch adds constructor and destructor to thread_info.

gdb:

2017-03-29  Yao Qi  <yao.qi@linaro.org>

	* gdbthread.h (struct thread_info): Declare constructor and
	destructor.  Add some in-class member initializers.
	* thread.c (free_thread): Remove.
	(init_thread_list): Call delete instead of free_thread.
	(new_thread): Call thread_info constructor.
	(thread_info::thread_info): New function.
	(thread_info::~thread_info): New function.
	(delete_thread_1): Call delete instead of free_thread.
	(make_cleanup_restore_current_thread): Move tp and frame to
	inner block.