qemu: add architecture dependencies for host qemu

The host-qemu package currently builds the user mode emulation, which
isn't available on all architectures, so this commit adds the
necessary architecture dependencies to the Config.in.host file.

Fixes:

  http://autobuild.buildroot.net/results/4298ce40cbe3db50ffe53ecf6856523157df591d/



Reported-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>