Centralize the handling of install_name and rpath.

This centralizes the Makefile handling of -install_name and -rpath. It also
moves the cmake build to using @rpath. The reason being that libclang needs it,
and it works for everything else.

A followup patch will move clang to using this and then there will be a single
point to edit to support other systems.

llvm-svn: 202499