use C11's _Static_assert() for BUILD_BUG_ON etc when available

use C11's _Static_assert() for BUILD_BUG_ON etc when available

This produces a more meaningful compiler diagnostic in case it happens
to trigger.

As a minor piece of cleanup, at once also use BUILD_BUG_ON_ZERO() to
actually define BUILD_BUG_ON() in the legacy case, slightly reducing
redundancy.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>