x86: disable Voyager temporarily

x86/Voyager does not build right now and it's unclear whether it will
be cleaned up and ported to the subarch-less 32-bit x86 code - so disable
it for now.

If it's fixed we'll re-enable it - or remove it after some time. There's
a very low number of systems running development kernels on x86/Voyager
currently. (one or two on the whole planet)

Signed-off-by: Ingo Molnar <mingo@elte.hu>