m68k/irq: Rename {,__}m68k_handle_int()

  - Rename m68k_handle_int() to generic_handle_irq(), and drop the unneeded
    asmlinkage,
  - Rename __m68k_handle_int() to do_IRQ().

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>