DMD as cross-compiler

[Walter Bright]

I realized that DMD is pretty close to being a cross-compiler, and won't need 
much work to go the rest of the way. I anticipate it could be very convenient 
when inconvenient to get the native DMD installed. It also should be useful for 
debugging problems with test suite failures :-/

Much of the static versioning in the backend is the result of converting it from 
C with its #if statements. Being done with all that preprocessor legacy will be 
nice.

Working on a series of incremental PRs to make this happen.