Set the default value for MaxAtomicSizeInBitsSupported to 0.

This was planned since its introduction, but wasn't rolled out for a
little bit longer than intended (ahem...8 years).

All in-tree targets have now been adjusted to call
setMaxAtomicSizeInBitsSupported explicitly where required, so this
should be a no-op. The docs in docs/Atomics.rst already claimed the
default was 0, so that doesn't need updating.