stubdom: make GMP aware that it's being cross-compiled

stubdom: make GMP aware that it's being cross-compiled

Append --build and --host flags to GMP's configure script so that it
knows it is being cross-compiled.

This should fix the issue that GMP doesn't compile with gcc 6, because
configure script won't try to test the host environment anymore.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>