Remove usages of find_inferior in linux-arm-low.c
    
    Replace two usages with the overload of for_each_thread that filters on
    pid.  It allows to simplify the callback a little bit.
    
    gdb/gdbserver/ChangeLog:
    
    	* linux-arm-low.c (struct update_registers_data): Remove.
    	(update_registers_callback): Return void, take arguments
    	directly, don't check thread's pid.
    	(arm_insert_point, arm_remove_point): Use for_each_thread.