linux-user: Fix sched_get/setaffinity conversion

sched_get/setaffinity linux-user syscalls were missing conversions for
little/big endian, which is hairy since longs may not be the same size
either.

For simplicity, this just introduces loops to convert bit by bit like is
done for select.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180109201643.1479-1-samuel.thibault@ens-lyon.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
