HIP: Try to deal with more llvm package layouts

The various HIP builds are all inconsistent.

The default llvm install goes to ${INSTALL_PREFIX}/bin/clang, but the
rocm packaging scripts move this under
${INSTALL_PREFIX}/llvm/bin/clang. Some other builds further pollute
this with ${INSTALL_PREFIX}/bin/x86_64/clang. These should really be
consolidated, but try to handle them for now.