Add option UNIFIED_OBJDIR, on by default, which moves the default build OBJDIR.

This changes the build OBJDIR from the older style of /usr/obj/<srcdir> for
native builds, and /usr/obj/<target>.<target_arch>/<srcdir> for cross builds to
a new simpler format of /usr/obj/<srcdir>/<target>.<target_arch>. This
new format is used regardless of cross or native build. It allows
easier management of multiple source tree object directories.

The UNIFIED_OBJDIR option will be removed and its feature made permanent
for the 12.0 release.

Relnotes: yes (don't note UNIFIED_OBJDIR option since it will be removed)
Prior work: D3711 D874
Reviewed by: gjb, sjg
Discussed at: https://lists.freebsd.org/pipermail/freebsd-arch/2016-May/017805.html
Discussed with: emaste
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12840