Fixes failed compile with [-Werror=implicit-fallthrough=], which is
default on gcc-7.x on distributions like fedora.

Enabling no implicit fallthrough for more than just clang as well to
catch this going forward.

Change-Id: I6cd880dac70ec126bd7812e2d9e5ff804d32cadd
Signed-off-by: Vincent Batts <vbatts@redhat.com>
Reviewed-on: https://boringssl-review.googlesource.com/20564
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>