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>