[CMake] Refactor and cleanup generating and installing symlinks to tools.

Summary: This change generalizes symlink generation and makes symlinks to tools obey LLVM_TOOLCHAIN_TOOLS. It makes it so that if you exclude llvm-ar from LLVM_TOOLCHAIN_TOOLS you don't end up with broken symlinks to llvm-lib and llvm-ranlib in your install.

Reviewers: bogner, chapuni, rafael

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12864

llvm-svn: 247632