More ppc64-only function wrapping hacks:

More ppc64-only function wrapping hacks:

- increase size of redirect stack from 8 to 16 elems

- augment the _NRADDR pseudo-register with _NRADDR_GPR2,
  which is the value of R2 at the most recent divert point.
  This is needed in the ELF ppc64 ABI in order to safely run
  the function being wrapped.

- add pseudo-instruction to read get _NRADDR_GPR2 into _GPR3.

- related change: always keep R2 up to date wrt possible memory
  exceptions (no specific reason, just being conservative)

git-svn-id: svn://svn.valgrind.org/vex/trunk@1543