Fix compilation of OS X handler on PPC with 10.5 SDK

With the 10.5 SDK, the ppc_thread_state_t struct prefixes register names with
__, just like the x86 struct. This patch just switches the PPC code paths to use
the REGISTER_FROM_THREADSTATE macro that we're already using for x86, and tweaks
the ifdefs around it to do the right thing in these situations. This builds fine
for me in a ppc/x86 universal build against the 10.4 and 10.5 SDKs.

Neal, this looks like it conflicts with your patch over on
http://breakpad.appspot.com/24001/show , but it accomplishes the same aim, I
think. Can you take a look? We checked in something similar to this in our
source tree just to fix PPC breakage.

[mochalatte]

LGTM