Clang segfault on rpi3 workaround using -O2

[bob prohaska]

The segfaults in clang during buildworld on rpi3 persisted past
r339376, but a hint from 
https://github.com/android-ndk/ndk/issues/386
suggested trying CFLAGS=-O2 as a long-shot workaround.

To my surprise, it worked. Three passes were required to
clear the segfaults, but now a -j4 buildworld/buildkernel
cycle runs without error to completion. A test of www/chromium
is underway now, and about half finished with no errors yet,
using r344113.

The github report is more than a year old and notes that
clang was fixed around that time. Is it possible the fix
didn't find its way into FreeBSD?

Thanks for reading,

bob prohaska