Makefile: Skip the meson subdir in cscope/TAGS/ctags

If the meson submodule is present, we don't really want to index its
source code. Consolidate the find command in a single place and use
it for cscope, ctags and etags. Note that this now causes ctags and
etags to also index assembly files, but this is okay since they both
have been supporting assembly since 2001 at least.

Signed-off-by: Greg Kurz <groug@kaod.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <159916247553.691541.10480293747685886851.stgit@bahia.lan>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>