build: Make GCC ignore fallthrough comments

build: Make GCC ignore fallthrough comments

From the GCC manpage:

> Wimplicit-fallthrough=5 doesn't recognize any comments as
> fallthrough comments, only attributes disable
> the warning.

So, check for the =5 version after checking for the simple version. This
way we get -Wfallhrough with clang and -Wfallthrough -Wfallthrough=5
with GCC, which works.