vc4: add meson option to disable optional neon support

Not all toolchains are able to compile the runtime optional vc4 neon support, so add an meson option to force disabling it at compile time.

For a failing example see e.g. [1], [2].

[1] http://autobuild.buildroot.net/results/6387b0a99e1a0922811919623d9a10b0943988df/build-end.log

[2] http://lists.busybox.net/pipermail/buildroot/2020-February/275304.html