Investigate speed improvements in 64-bit builds

In a comment on #4008 (moved), tmpname0901 says,

A related quick-fix is to provide 64-bit Windows executables. (I'm assuming that most users in Unix environments build their own Tor binaries.) The OpenSSL crypto code is just plain faster when built for a 64-bit CPU (relative to the same CPU running a 32-bit binary), and 64-bit Windows systems are no longer a rarity. The cost in local system responsiveness while running a Tor relay on Windows would be greatly reduced by simply doing a 64-bit build of the same code that's being built now.

We should look into how much this would help us, for how many servers, and how hard it would be to pull it off.