Package mingw dependencies

Package rustc's mingw dependencies into Windows installer to avoid requiring a separate mingw install.

Closes #11782
@alexcrichton: I think this is ok for all platforms.  That'll be one less gcc dependency to worry about if we ever switch to direct linker invocation.
@thestinger: gcc still tells linker to load its' lto plugin, I guess on the off chance that one of the libraries being linked was compiled with lto.