Phobos unit testing uncovers a CPU bug

[Walter Bright]

Don wrote:
> The code below compiles to a single machine instruction, yet the results 
> are CPU manufacturer-dependent.

This is awesome work, Don. Kudos to you, David and Dmitry.

BTW, I've read that fine-grained CPU detection can be done, beyond what CPUID 
gives, by examining slight differences in FPU results. I expect that *, +, -, / 
should all give exactly the same answers. But the transcendentals, and obviously 
yl2x, vary.