Reserve a register for ppcState.

The register is RBP, previously in the GPR allocation order.  The next
commit will investigate whether there are too few GPRs (now or before),
but for now there is no replacement.

Previously, it was accessed RIP relatively; using RBP, anything in the
first 0x100 bytes of ppcState (including all the GPRs) can be accessed
with three fewer bytes.  Code to access ppcState is constantly generated
(mostly by register save/load), so in principle, this should improve
instruction cache footprint significantly.

I think I have measured this as significantly improving the povray time,
but I need confirmation.  Also should get numbers from Instruments on
whether instruction cache misses are actually being reduced.  Just
putting this out there for now.  *yawwn*

The vast majority of this commit is mechanically replacing
M(&PowerPC::ppcState.x) with a new macro PPCSTATE(x).