Revert "Use relative rpath so that the installation and build dirs are relocatable."

This reverts commit r201921.

This should bring the polly bots back. I will try to build it locally to
understand how cmake was setting the rpath of LLVMPolly.so.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201934 91177308-0d34-0410-b5e6-96231b3b80d8