docs: submit-checklist: Expand on build tests against different word sizes

Existing sentence on cross-compilation that mentions ppc64 does not
make much sense in today's perspective.

Expand it for the benefits of testing against architectures of
different word sizes and endianness.

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/05c0b99c-c2e9-4702-90fd-8a4127586424@gmail.com