[llvm] r201921 - Use relative rpath so that the installation and build dirs are relocatable.

[Rafael Espíndola]

> This patch is exactly what I've tried.
>

Cool. The only use of @rpath in the old build system was in
tools/llvm-shlib/Makefile, while -Wl, at executable_path/../lib was used
in Makefile.rules, so I have changed the cmake build in r202302.

Cheers,
Rafael