Use fallthrough attribute.

Use fallthrough attribute.

Use __attribute__ ((fallthrough)) to indicate switch case fall through
instead of a comment. This ensures that the fallthrough warning is not
triggered even if the file is pre-processed (hence stripping the
comments) before it is compiled.

The actual fallback implementation is hidden behind a FALLBACK macro in
case the compiler doesn't support it.

Finally, the -Wimplict-fallthrough warning was upgraded to only allow
the attribute to satisfy it; a comment alone is no longer sufficient.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>