meson.bbclass: add cython binary to cross/native toolchain config

This allows building Cython based Python modules with the native
meson support which has been present since meson version 0.59.

https://mesonbuild.com/Cython.html



Signed-off-by: Jack Mitchell <ml@embed.me.uk>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>