[PATCH RFC v2] s390: convert to GENERIC_VDSO

[Sven Schnelle]

as discussed here's the second version of the generic vdso patch for s390.

Changes in v2:
- added patch from Thomas that adds vdso_update_begin()/vdso_update_end()
- changed the s390 code to use the vdso update functions
- changed the name of the architecture specific data to 'arch_data'