[runtimes] Correctly apply libdir subdir for multilib (#93354)

We weren't applying the libdir subdir to header directories but this is
necessary for correctness when building e.g. ASan variant. This change
also updates path construction logic accross all runtimes and ensures
they're consistent.

NOKEYCHECK=True
GitOrigin-RevId: ed155f3f237a7e3a3e40a84d815225d27e83f0db
1 file changed