[Issue 18907] Support cross-compiling

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18907

--- Comment #1 from Manu <turkeyman at gmail.com> ---
I should also add, this would be very useful for developing, but not JUST for
developing.

I have a couple of projects where we generate our windows builds from linux CI
infrastructure. We cross-compile our windows builds from linux, and DMD can't
currently participate in that workflow.
I have used wine to build for windows using DMD from linux in the past, and
that is... definitely less than ideal.

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