syscalls: 'define-c-struct' supports cross-compilation.

Reported by Jan (janneke) Nieuwenhuizen <janneke@gnu.org>.

Before that, we'd always use the 'sizeof' and 'alignof' value obtained
from the host at macro-expansion time.

* guix/build/syscalls.scm (sizeof*, alignof*): When the target word size
differs from the host word size, emit a call to 'sizeof'/'alignof'.