Take LLVM_TOOLS_DIR from not based on --obj-root, but from --bindir directly. "llvm-config --bindir" knows appropriate prefix.

It's followup since llvm r197684.

git-svn-id: https://llvm.org/svn/llvm-project/dragonegg/trunk@197742 91177308-0d34-0410-b5e6-96231b3b80d8