Add objc to the Meson cross file as well as detection of Darwin.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Joelle van Dyne <j@getutm.app>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210126012457.39046-8-j@getutm.app
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>