s390/vdso: always enable vdso

With the upcoming move of the svc sigreturn instruction from
the signal frame to vdso we need to have vdso always enabled.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>