[PowerPC] Add new Future CPU for PowerPC

This patch will add -mcpu=future into clang for PowerPC.

A CPU type is required for work that may possibly be enabled for some future
Power CPU. The CPU type future will serve that purpose. This patch introduces
no new functionality. It is an incremental patch on top of which Power PC work
for some future CPU can be done.

Differential Revision: https://reviews.llvm.org/D70262