meson: compile build tool gen_matypes for the build machine

Closed Eric Engestrom requested to merge eric/mesa:meson-gen_matypes into master

Meson 0.51+ warns: Target gen_matypes is used as a generator, but is built for the host machine. This means most cross builds will fail. You might want to set native: true instead to build it for the build machine.

/cc @dbaker

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