Package mingw dependencies

Package rustc's mingw dependencies into Windows installer to avoid requiring a separate mingw install.

Closes #11782
Ah I just realize that this may yield surprising results when dealing with usage like `rustdoc --test`. In that case each test is compiled within the same process, which means that `PATH` will be appended to repeatedly (which may end badly).

Could this perhaps be wrapped in a `Once` or something similar?