s390/vdso: map vdso also for statically linked binaries

s390 does not map the vdso for statically linked binaries, assuming
that this doesn't make sense. See commit fc5243d9

 ("[S390]
arch_setup_additional_pages arguments").

However with glibc commit d665367f596d ("linux: Enable vDSO for static
linking as default (BZ#19767)") and commit 5e855c895401 ("s390: Enable
VDSO for static linking") the vdso is also used for statically linked
binaries - if the kernel would make it available.

Therefore map the vdso always, just like all other architectures.

Reported-by: Stefan Liebler <stli@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>