[PowerPC] Update /platforms to reflect the powerpc* switch to clang.

[bdragon]

Summary

As of rS356111, the default compiler on all powerpc* targets in HEAD is no longer GCC4.

As such, remove powerpc* from the automatic-transition-to-tier-4 list.

(This includes powerpcspe, which is currently broken as-is in HEAD because of some target triple issues that will be fixed soon, but builds and works fine in clang with some minor out-of-tree patches.)

[bdragon]

Not sure who else I should be tagging on this. Almost added core as a group reviewer but changed my mind at the last minute, this seems too obvious and straightforward of a change to waste their time.

[jhibbits]

Add docs for review.

[bcr]

OK from docs.