MFC: Rework the support used by ABIs to override resource limits so that
a 64-bit process exec'd by a 32-bit process doesn't end up with 32-bit
limits.

This doesn't break the ABI as neither of the 32-bit ABIs (COMPAT_LINUX32
and COMPAT_IA32) are buildable as modules on 6.x/amd64 and none of the
other ABIs use this hook.