[llvm][cmake] Make `llvm_install_symlink` robust to absolute dirs.

If `CMAKE_INSTALL_BINDIR` is a different absolute path per project, as
it is with NixOS when we install every package to its own prefix, the
old way fails when the absolute path gets prepended.

There are still some issues with dowstream packages using `LLVM_TOOLS_INSTALL_DIR` which also may be absolute and just for LLVM proper, but that will be addressed in a future commit.

Differential Revision: https://reviews.llvm.org/D101070