add "%" prefix to register names in assembly code.
change assembly functions to return values in %a0 instead of %d0.
C symbols no longer prepend an underscore, adjust assembly code for this.
32-bit values are now 32-bit aligned instead of 16-bit aligned,
adjust structure packing and padding to override this where necessary.
make EXEC_ELF std, make EXEC_AOUT and COMPAT_AOUT_M68K optional.
use the MI loadfile() instead of several home-grown versions.
