[PING, needs GWP?] Use UNSPECs for Darwin PIC references

[Richard Sandiford]

http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01535.html
Use UNSPECs for Darwin PIC references.  As described in the
message, I think this is an improvement in its own right.
It would also allow us to remove a piece of simplify-rtx.c
code that is causing regressions on some targets.

FWIW, I pinged Stan Shebs about this privately a week or so ago,
but heard nothing back.  I believe he's the only non-Apple
Darwin maintainer.  (He might simply be on holiday, of course.)

David has already approved the ppc parts of this patch series
(thanks).

Richard