[Clang] Make '-frtlib-add-rpath' include the standard library directory (#86217)

Summary:
The original intention of the `openmp-add-rpath` option was to add the
rpath to the language runtime directory. However, the current
implementation only adds it to the compiler's resource directory. This
patch adds support for appending the `-rpath` to the compiler's standard
library directory as well. Currently this is `<exe>/../lib/<triple>`.