Improve target triple options

[tritao]

Right now we support cross generation for all platforms and architectures but this is not documented or easy to figure out.

Improve the API by extend the existing Triple class and changing options.TargetTriple.

Triple(string),
Triple(TargetPlatform, TargetArch) ctors

[fsinisi90]

Is this documented anywhere already? I want to generate Windows bindings on macOS but I have no idea where to start.

[ddobrev]

@fsinisi90 if you need the correct target triple for your settings, for the time being you'll have to construct it yourself. https://clang.llvm.org/docs/CrossCompilation.html documents how.