Use gdbarch for probe::get_argument_count
    
    The probe function get_argument_count does not need a frame, only
    the current gdbarch.  Switch the code to pass gdbarch instead.
    No functional changes.
    
    gdb/ChangeLog:
    
    	* break-catch-throw.c (fetch_probe_arguments): Use gdbarch.
    	* dtrace-probe.c (dtrace_probe::get_argument_count): Likewise.
    	* probe.c (probe_safe_evaluate_at_pc) (compute_probe_arg)
    	(compile_probe_arg): Likewise.
    	* probe.h (get_argument_count): Likewise.
    	* solib-svr4.c (solib_event_probe_action): Likewise.
    	* stap-probe.c (stap_probe::get_argument_count): Likewise.