Don't write to inferior_ptid in remote.c
    
    gdb/ChangeLog:
    2020-06-18  Pedro Alves  <palves@redhat.com>
    
    	* remote.c (remote_target::remote_notice_new_inferior): Use
    	switch_to_thread instead of writing to inferior_ptid directly.
    	(remote_target::add_current_inferior_and_thread): Use
    	switch_to_no_thread instead of writing to inferior_ptid directly.
    	(extended_remote_target::attach): Use switch_to_inferior_no_thread
    	and switch_to_thread instead of using set_current_inferior or
    	writing to inferior_ptid directly.