Chez for architectures without native backends

It looks like the problem might be the assumption in https://github.com/racket/racket/blob/ab6dc20f78ff4e75655f245e28de44fa62a4b82d/racket/src/cs/c/configure.ac#L447-L456:

# For a pb build where Racket is supplied, force cross-build
# mode on the assumption that the host is not a pb build
# (because it should be created with default configure options)
if test "${enable_pb}" = "yes" ; then
  if test "${enable_racket}" != "" ; then
    if test "${enable_target}" = "" ; then
      enable_target="${MACH}"
    fi
  fi
fi