libamd_comgr does not set its RPATH properly

[stellaraccident]

LLVM is generally well behaved with RPATHs so we don't touch them, but there is a helper that needs to be used. It appears that amd_comgr is not doing this, so it can't find libLLVM at runtime.

In the super-project, we're fixing this after the fact in compiler/amd-llvm/post_hook.cmake, but it would be better to do this upstream.