acpi: add build_append_gas() helper for Generic Address Structure
it will help to add Generic Address Structure to ACPI tables
without using packed C structures and avoid endianness
issues as API doesn't need an explicit conversion.
Signed-off-by: Igor Mammedov <imammedo@redhat.com> Reviewed-by: Eric Auger <eric.auger@redhat.com> Tested-by: Eric Auger <eric.auger@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>