The entire contents of the header is host-specific, but the
existence of such a header is not, which could prevent some
host specific ifdefs at the top of the file for the include.

Add host/include/{arch,generic} to the project arguments.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>