Add support for rdrand Ivy Bridge instruction

[brson]

When running on Ivy Bridge chips our rand module can just defer to the dedicated random number generator.

[pnkfelix]

visited for bug triage on 2013 july 2.

[huonw]

I'm going to include this as part of my std::rand rewrite, which I'm hoping to get mergeable in the next week or two.

[catamorphism]

@huonw Did this end up landing yet?

[huonw]

Not yet.

[brson]

I'm not entirely sure we should just defer random number generation to hw. The linux kernel at least does not trust it, and just uses it as one entropy source of many.

[huonw]

@brson I agree in light of the recent NSA + RDRAND scandal & the linux kernel's opinion of it. We can just defer to the OS for "strong" random numbers and let it decide about the quality of whatever sources of randomness it has, so we don't have to keep up with which vendors have trustworthy CPU RNGs etc.

(FWIW, I tried implemented this, but I never quite got the asm! to work 100% reliably.)

Closing this as wontfix, feel free to reopen if someone really thinks this is necessary.