configure: Compile with -Wimplicit-fallthrough=2

Coverity always complains about switch-case statements that fall through
the next one when there is no comment in between - which could indicate
a forgotten "break" statement. Instead of handling these issues after
they have been committed, it would be better to avoid them in the build
process already. Thus let's enable the -Wimplicit-fallthrough warning now.
The "=2" level seems to be a good compromise between being too strict and
too generic about the possible comments, so we'll start with "=2" for now.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20201211152426.350966-13-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>