This commit fixes the syntax of the implicit fallthrough comments, in
order to be matched by GCC (and probably other compilers as well) with
the following regular expression:
[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )? fall(s | |-)?thr(ough|u)[
\t.!]*(-[^\n\r]*)?
See: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wimplicit-fallthrough=
