Rework r254951 to not reach into private OBJDIR areas.

The original change was dealing with the build wanting to run a newer
install(1) that was not yet installed. The solution to look into the private
legacy directory of the existing build conflicts with 2 upcoming features: a
changed OBJDIR format, and splitting the host tools into arch-dependent and
arch-independent directories. Rather than hardcoding and changing the paths in
this script, just let kernel-toolchain do the work, while disabling much of the
meat. With -j15 this finishes in 25 seconds for me and 117 seconds with -j1.
All that is really needed is bootstrap-tools, but the system is not currently
written in a way that all previous dependent steps will have ran. The previous
steps, such as _worldtmp, are being reworked and renamed and so cannot be
relied upon to be right.

Sponsored by: Dell EMC Isilon