configure: tell meson and contrib_plugins about DLLTOOL

To cleanly handle cross-building we need to export the details of
dlltool into meson's list of cross binaries and into the
contrib/plugins/ make configuration.

Cc: Greg Manning <gmanning@rapitasystems.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231106185112.2755262-13-alex.bennee@linaro.org>