Implement --long-plt flag (ARM only).

Implement --long-plt flag (ARM only).

gold/
    PR gold/18780
    * arm.cc (Target_arm::do_make_data_plt): Choose PLT generator based
    on value of --long-plt flag.
    (Output_data_plt_arm_standard::do_get_plt_entry_size): Moved to
    Output_data_plt_arm_short.
    (Output_data_plt_arm_standard::do_fill_plt_entry): Likewise.
    (Output_data_plt_arm_standard::plt_entry): Likewise.
    (Output_data_plt_arm_standard::do_fill_first_plt_entry): Fix
    variable reference.
    (Output_data_plt_arm_short): New class.
    (Output_data_plt_arm_short::do_fill_plt_entry): Error out on too large
    PLT offsets instead of asserting.
    (Output_data_plt_arm_long): New class.
    * options.h (General_options): Define --long-plt flag.