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S/390 modernization tasks

At the moment, s390-tdep.c uses quite a few deprecated gdbarch
methods.  Here's a list of the changes currently needed to bring that
code forward.  So much has changed that I'm sure some of the
descriptions of what needs to be done aren't exactly right, but at the
very least, I think they'll point to areas that need attention.

- The way a target maps out its register set has changed.

  These old methods don't seem to be necessary any more:

  The new method gdbarch_register_type seems to replace:
  - gdbarch_pseudo_register_read
  - gdbarch_pseudo_register_write

- Some interfaces now use regcaches instead of byte arrays.

  A number of gdbarch methods have been converted from using a raw
  registers array (or the equivalent) to using the more opaque
  regcache structure.  These include:
  - EXTRACT_STRUCT_VALUE_ADDRESS (which the s390 can't do anyway)

- There is a new, much simpler interface for frame unwinding.

  One simply registers a function that, given a frame's PC, returns a
  structure of pointers to functions that can unwind that frame's
  registers.  This seems to replace a number of functions:

- The generic_find_dummy_frame function now returns a regcache.

  The functions s390_frame_saved_pc_nofix and s390_frame_chain use
  deprecated_generic_find_dummy_frame, which returns an array of bytes
  containing values for the registers saved in the dummy frame.  The
  new generic_find_dummy_frame interface returns a regcache instead.

  The functions that use generic_find_dummy_frame will all go away (I
  think) under the new frame unwinding system, so this may not need
  attention if that's done first.  However, it is a quick improvement
  that one could get out of the way before undertaking the larger

- There is a new inferior function call interface.

  The new push_dummy_call gdbarch method combines all the following
  into one call with a lot of arguments:

- The DEPRECATED_PC_IN_CALL_DUMMY function isn't necessary any more.

  Each dummy frame structure now holds the address at which its return
  breakpoint is set; this means that generic code (specifically,
  pc_in_dummy_frame) can recognize PC's at call dummy breakpoints.

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