Previously, `mi_atomic_yield` would not be defined on ARM32 big-endian architectures if they were pre-ARMv7. Rework the #ifdef guard to be more readable and collapse the ARM guards so both little and big endian are handled via the same mechanism. Now, ARMv7+ will utilize `yield` while older targets will use `nop` regardless of endianness. Signed-off-by: Vincent Fazio <vfazio@gmail.com>