powerpc/kexec: Add kexec "hold" support for Book3e processors

Motivation:
IBM Blue Gene/Q comes with some very strange firmware that I'm trying to get out
of using in the kernel.  So instead I spin all the threads in the boot wrapper
(using the firmware) and have them enter the kexec stub, pre-translated at the
virtual "linear" address, never touching firmware again.

This works strategy works wonderfully, but I need the following patch in the
kexec stub. I believe it should not effect Book3S and Book3E does not appear
to be here yet so I'd love to get any criticisms up front.

This patch adds two items:

1) Book3e requires that GPR4 survive the "hold" process, so we make
   sure that happens.
2) Book3e has no real mode, and the hold code exploits this.  Since
   these processors ares always translated, we arrange for the kexeced
   threads to enter the hold code using the normal kernel linear mapping.

Signed-off-by: Jimi Xenidis <jimix@pobox.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>