locking/atomic: remove ARCH_ATOMIC remanants

Now that gen-atomic-fallback.sh is only used to generate the arch_*
fallbacks, we don't need to also generate the non-arch_* forms, and can
removethe infrastructure this needed.

There is no change to any of the generated headers as a result of this
patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210713105253.7615-3-mark.rutland@arm.com