Dump register notes for each thread when generating a FreeBSD core.

Dump register notes for each thread when generating a FreeBSD core.

gdb/ChangeLog:

	* fbsd-tdep.c (find_stop_signal): Remove.
	(struct fbsd_collect_regset_section_cb) <lwp>: New field.
	<stop_signal>: New field.
	<abort_iteration>: New field.
	(fbsd_collect_regset_section_cb): Use new fields.
	(fbsd_collect_thread_registers): New function.
	(struct fbsd_corefile_thread_data): New structure.
	(fbsd_corefile_thread): New function.
	(fbsd_make_corefile_notes): Use new function to dump notes for each
	non-exited thread in a process.