Avoid compiler warnings on 64-bit native Windows.

Reported by Wilfredo Velázquez-Rodríguez at
<https://savannah.gnu.org/bugs/?68178>.

* lib/genaliases.c (emit_encoding): Emit a cast to size_t, not 'long'.
* lib/genaliases2.c (emit_encoding): Likewise.
* lib/iconv.c (sysdep_aliases): Cast to size_t, not 'long'.