On a whim, the system was updated to r343731 and it successfully
built the toolchain and a new kernel.  One major surprise was that
make -j4 kernel-toolchain took around twelve hours, rather than the
few hours observed on previous attempts. 

The build procedure was identical: Update source, clean out the
working directories, make a kernel toolchain, make a kernel and
reboot, then remake world and kernel.

The difference was that after the new kernel was booted, working
directories were again cleaned. This imposes a considerable time
penalty, around twelve hours, on the final build cycle.