[clang] Don't assume location of compiler-rt for OpenBSD (#92183)

If the `/usr/lib/...` path where compiler-rt is conventionally installed
on OpenBSD does not exist, fall back to the regular logic to find it.

This is a minimal change to allow OpenBSD cross compilation from a
toolchain that doesn't adopt all of OpenBSD's monorepo's conventions.

(cherry picked from commit be10746f)