Add $_gthread convenience variable

Add $_gthread convenience variable

This commit adds a new $_gthread convenience variable, that is like
$_thread, but holds the current thread's global thread id.

gdb/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention $_gthread.
	* gdbthread.h (struct thread_info) <global_num>: Mention
	$_gthread.
	* thread.c (thread_num_make_value_helper): New function.
	(thread_id_make_value): Delete.
	(thread_id_per_inf_num_make_value, global_thread_id_make_value):
	New.
	(thread_funcs): Adjust.
	(gthread_funcs): New.
	(_initialize_thread): Register $_gthread variable.

gdb/testsuite/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.base/default.exp: Expect $_gthread as well.
	* gdb.multi/tids.exp: Test $_gthread.
	* gdb.threads/thread-specific.exp: Test $_gthread.

gdb/doc/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Threads): Document the $_gthread convenience
	variable.
	(Convenience Vars): Likewise.