Ensure that arch.test for host runs with option(host_build)

So that mkspecs and features may rely on the host_build test.

Change-Id: I18fee4820d9e2904285afcc7ddb8f1cc3d025fef
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>