Add x64 and x86_64 to user agent regex

[aaronfranke]

Fixes #1467

As noted in the issue, ARM is explicitly not included here. Just WOW64, Win64, x64, and x86_64.

[peff]

Thanks, this seems like an obvious and easy improvement, even if we later go further with ARM, moving the check to client-side, etc.

[peff]

I don't think we have any real way of testing this meaningfully. Presumably it works for you, but the big question is whether anybody would see a false positive. I tend to doubt it based on those patterns, so I'll merge and we'll keep an eye open for anybody opening an issue (but again, I'd be surprised if we see one).

[peff]

One interesting implication: on my 64-bit linux machine I now get the 64-bit windows download (I have to navigate manually to /download/win, of course). And those two things may or may not be correlated, since I'm clearly not downloading for my current platform. 😉 I think that's the best we can do for the auto-download, though.