RPI3 clang crashes in buildworld

[bob prohaska]

Has anyone had recent success running buildworld on an RPI3? I'm seeing
signal 11's with increasing regularity during the building libraries phase.
Two months ago -j4 buildworld ran successfully.

It's possible my system is corrupted. I've tinkered with  a kernel patch at
https://people.freebsd.org/~gonzo/arm/patches/vchiq-wip-20180217.diff
back when the system was at r342781, but the patch was reversed and the
machine is now up to r343165 using "clean start" buildworld without -j
and careful restarts, deleting the last .o file before the crashes.

There are no hardware errors on the console and only clang is crashing.
The machine will even run the chromium browser without crashing, but it
does grumble:

bob at www:~ % chrome
[83827:1218383872:0120/075747.927080:ERROR:gpu_process_transport_factory.cc(1016)] Lost UI shared context.
[84111:1339003392:0120/075753.574384:ERROR:command_buffer_proxy_impl.cc(113)] ContextResult::kFatalFailure: Shared memory handle is not valid 

The most recent buildworld steps were 
svnlite up /usr/src
make cleandir (twice)
rm -rf /usr/obj/usr/
make kernel-toolchain
make buildkernel
make installkernel
(reboot)
make buildworld (using -DNO_CLEAN for restarts)

Have I taken all the steps possible for a clean rebuild?

At the moment both buildworld and chromium are running, without 
any additional problems.

Thanks for reading,

bob prohaska