Lowest i386 CPU Model with proper C++ atomics

[Florian Weimer]

* Joel Sherrill:

> I don't know that we have a huge issue in making the i486 a minimum.
> I was proposing a Pentium II or P6 as a baseline since that moves you
> up to having a TBR and initial SMP support.

Sorry, what's a TBR?

> But I think there are still embedded x86 clones that I am not sure
> meet the P6 minimum.

Some AMD Geode variants do not have CMOV and have been produced until
fairly recently (if they aren't in production still).  That means that
they do not meet the Pentium Pro baseline.