Add ppcg-0.04 to lib/External

ppcg will be used to provide mapping decisions for GPU code generation.

As we do not use C as input language, we do not include pet. However, we include
pet.h from pet 82cacb71 plus a set of dummy functions to ensure ppcg links
without problems.

The version of ppcg committed is unmodified ppcg-0.04 which has been well tested
in the context of LLVM. It does not provide an official library interface yet,
which means that in upcoming commits we will add minor modifications to make
necessary functionality accessible. We will aim to upstream these modifications
after we gained enough experience with GPU generation support in Polly to
propose a stable interface.

Reviewers: Meinersbur

Subscribers: pollydev, llvm-commits

Differential Revision: http://reviews.llvm.org/D22033

llvm-svn: 275274