m68k/atari: Initial ARAnyM support

Add improved support for running under the ARAnyM emulator
(Atari Running on Any Machine - http://aranym.org/).

[michael, geert: Cleanups and updates]

Signed-off-by: Petr Stehlik <pstehlik@sophics.cz>
Signed-off-by: Michael Schmitz <schmitz@debian.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>