configure: add `--enable-natipkg` and 64-bit Linux native libraries

The `--enable-natipkg` configuration option adds "-natipkg" to the
platform library subpath. The suffix is intended to trigger the
installation of packages that supply native libraries for supported
platforms (where 64-bit Linux is the supported platform, for now, for
main-distribution packages), instead of relying on libraries installed
via the OS's package manager.

The intended client for "-natipkg" is the package-build service, where
installing packages via the OS package manager would require network
access and either trust or constrained installations. The build
machine is intentionally disconnected from the network and can only
access Racket packages, so repackaging native libraries as Racket
packages makes those libraries accessible.

A disadvantage of this approach to installing native libraries is that
it creates work for implementers of packages that access native
libraries. Those implementers will have to supply packages for 64-bit
Linux versions of native libraries to the degree needed to build and
(eventually) test the package. An advantage of the approach is that it
requires no changes to the package system; it will be cheap to replace
this approach if we find a better way to deal with native libraries
and/or OS packages in the package-build service.