Root the lib32 object tree under the overall object tree.

This enables a common root directory for all object files for a given tree,
which eases sharing a common MAKEOBJDIRPREFIX, and cleaning up of object trees.

In particular, one can simply (from the source directory) rm -rf /usr/obj$(pwd)
to destroy all object files for it. Or to copy/sync files, etc.

Reviewed by: bdrewery
CR: https://reviews.freebsd.org/D796
MFC after: 1 month
Sponsored by: Spectra Logic