Specifying a default rpath (noob question)

Hi, I’m building a toolchain residing in a non-standard location and thus I’d like to override the default rpath of every executable later produced by the resulting clang / lld.
With GCC there was an arcane --with-specs='%{!static:%x{-rpath=/custom/path}}', is there a way to do the same for clang?

Tried config files, turns out their discovery relies on reading /proc/self/exe which I don’t have. Besides, there doesn’t seem to be a way to specify directives that only affect dynamic linking.