s390/vdso: rename VDSO64_LBASE to VDSO_LBASE

Will be used by both vdso32 and vdso64, so change the name.

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>