Changes to make function wrapping work better on ppc64-linux:

Changes to make function wrapping work better on ppc64-linux:

- when recording the non-redirected address in guest_NRADDR, also
  snapshot the current R2 value, as that will be needed to run the
  original safely

- As a consequence, the original-function information extracted by
  VALGRIND_GET_ORIG_FN is different on ppc64-linux (2 words) from
  all other platforms (1 word).  So change the type of it from
  void* to a new type OrigFn which can be defined differently for
  each platform.

- Change the CALL_FN_* macros for ppc64-linux to save/restore
  R2 values appropriately.

- ppc64-linux: detect overflow/underflow of the redirect stack
  and bring Valgrind to a halt if this happens

- Update VG_CLREQ_SZB for ppc32/64 (was out of date).

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@5569