A set oftimekeeping/VDSO updates:

 - Preparatory work to allow S390 to switch over to the generic VDSO
   implementation.

   S390 requires that the VDSO data pointer is handed in to the counter
   read function when time namespace support is enabled. Adding the pointer
   is a NOOP for all other architectures because the compiler is supposed
   to optimize that out when it is unused in the architecture specific
   inline. The change also solved a similar problem for MIPS which
   fortunately has time namespaces not yet enabled.

   S390 needs to update clock related VDSO data independent of the
   timekeeping updates. This was solved so far with yet another sequence
   counter in the S390 implementation. A better solution is to utilize the
   already existing VDSO sequence count for this. The core code now exposes
   helper functions which allow to serialize against the timekeeper code
   and against concurrent readers.

   S390 needs extra data for their clock readout function. The initial
   common VDSO data structure did not provide a way to add that. It now has
   an embedded architecture specific struct embedded which defaults to an
   empty struct.

   Doing this now avoids tree dependencies and conflicts post rc1 and
   allows all other architectures which work on generic VDSO support to
   work from a common upstream base.

 - A trivial comment fix.
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