Add llvm-link and clang to CMake install

[hcindyl]

They are used to generate ukernel bitcode targets. Include the binaries in the host tool install to support cross-compilation

The binaries will be added as CMake TARGET via iree_import_binary for cross-compile flows.

[ScottTodd]

There are other places using these CMake variables https://github.com/openxla/iree/blob/a623a93d27cd75b7c306a8282704f10b5a58ad3a/build_tools/cmake/iree_llvm.cmake#L74-L76 https://github.com/openxla/iree/blob/a623a93d27cd75b7c306a8282704f10b5a58ad3a/samples/custom_dispatch/cpu/embedded/CMakeLists.txt#L34-L39

I don't remember the full context here... but we should avoid having two different mechanisms for accessing the same tool

[hcindyl]

The change in this PR uses the variables set in iree_llvm.cmake. I can make the changes in samples for the consistency.

The goal is of this PR to make ukernel bitcode gen working in the cross-compilation scenarios, so clang and llvm-link needs to be in install/bin and iree_bitcode_library macro can use binaries from IREE_HOST_BIN_DIR` instead from the CMake target list.

[hcindyl]

I sorted out the mechanism: The binaries need to be added in CMake install, and to be added to iree_import_binary for the cross-compile flows, so they can be found with CMake <TARGET_FILE:target> syntax.