CPUTYPEs includig avx are suspicious with clang in stable/10

[Harry Schmalzbauer]

 I'm seeing sporadic problems (core dumps) with binaries compiled with
CPUTYPE=core-i-avx.
They run fine on IvyBridge, but fail on Haswells.
Stripping avx from MACHINE_CPU (by defining CPUTYPE=corei7 instead of
core-i-avx) solved all crashes on the haswell system.

I have no idea if applications like pkg(8) make use of AVX, I'd bet they
don't do. So I guess it's something wrong with clang.
Unfortuantely I currently don't have a development installation on any
haswell system to provide some useful backtraces, but perhaps somebody
with compiler knowledge can have a look at this problem?

Thanks,

-Harry