[llvm][cmake] Make `install_symlink` workflow work with absolute install 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 with `CMAKE_INSTALL_PREFIX`.

The `extend_path` function does what we want, but it is currently internal-only. So easier to just inline the one small case of it we need.

Also fix one stray `bin` -> `CMAKE_INSTALL_BINDIR`

Reviewed By: sebastian-ne

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