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

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>
(cherry picked from commit b1dcb1eb69032c30f5a8faf4d7120fc6c4ecd051)
Signed-off-by: Steve Sakoman <steve@sakoman.com>