Change libLTO back to linking with @executable_path instead of @rpath.

This partially reverts r187641 until ld64 adopts a change to link with an
rpath setting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194418 91177308-0d34-0410-b5e6-96231b3b80d8