CMake: Simplify llvm-config search

Make sure we pick up LLVM 3.3 if multiple versions are installed on the
system.

Signed-off-by: Pekka Enberg <penberg@iki.fi>
Use PPA for LLVM 3.3 packages

Use Niko Mikkilä's PPA LLVM 3.3 packages to build libcpu.

Signed-off-by: Pekka Enberg <penberg@iki.fi>