[PATCH] atomics: Remove unused atomics

[Wilco Dijkstra]

Hi Adhemerval,

> We will need to keep !USE_ATOMIC_COMPILER_BUILTINS for at least hppa and sh, where GCC 

It all works fine if I force USE_ATOMIC_COMPILER_BUILTINS=1 and fix m68k to not claim to have
64-bit atomics (it needs libatomic for load/store at least).

Cheers,
Wilco