Compat architectures that does not use generic unistd (mips, s390),
declare compat version in their syscall tables for preadv2 and
pwritev2. Generic unistd syscall table should do it as well.

[arnd: this initially slipped through the review and an
 incorrect patch got merged. arch/tile/ is the only architecture
 that could be affected for their 32-bit compat mode, every
 other architecture we support today is fine.]
Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>