[Gc] fetch-cas implementation for avr32 and m68k

[Ivan Maidanski]

Hello Bradley and Thorsten,

Could you please contribute AO_fetch_compare_and_swap_full implementation for avr32 and m68k targets, respectively, to libatomic_ops?

Thank you

Links:
https://github.com/ivmai/libatomic_ops/blob/master/src/atomic_ops/sysdeps/gcc/avr32.h  
https://github.com/ivmai/libatomic_ops/blob/master/src/atomic_ops/sysdeps/gcc/m68k.h  

Regards,
Ivan