Default to 64 bit compilation

[3246251196]

@Sarcasm I wonder if we should set the default CMAKE arguments to compile the server in 64bit land. There is nothing stopping the user from doing this themselves, but needing to remember to add the "-g Win64 15 2017" flag is somewhat cumbersome (if compiling for Windows /MSVC compiler).

I will close this ticket swiftly after I hear your opinion.

[Sarcasm]

Maybe just use irony-extra-cmake-args:

https://github.com/Sarcasm/irony-mode/blob/2b1db096b65260fa06690a81ce4e9fca2d164cdf/irony.el#L100-L103

I would gladly accept a PR that document this in the README, or maybe it can be added to the wiki.