[S390] Add set_fs(USER_DS) to start_thread().

Currently works anyway since search_binary_handler has a
set_fs(USER_DS). But start_thread() is the place where this should be
done. Following all other architectures...

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>