[NFC][PowerPC] Add the inheritable and additional features to make the processor...

[NFC][PowerPC] Add the inheritable and additional features to make the processor definition more clear

The old processor design assume that, all the old processor's feature must be
inherited into future processor. That is not true as instruction fusion or some
implementation defined features are not inheritable.

What this patch did:
  * Rename the old "specific features" to "additional features" that keep the new added inheritable features.
  * Use the "specific features" to keep those features only for specific processor.
  * Add the "inheritable features" to keep all the features that inherited from early processor.

Differential Revision: https://reviews.llvm.org/D70768