Create import library to go with dylib on Windows

[retep998]

On Windows, in order to link to a .dll you actually link to an import library instead, either .a or .lib. When creating a .dll with rustc --crate-type=dylib however, no such import library is created, thus making it difficult to link to from C/C++.
If an import library is created, the metadata could be moved from the .dll to the library, thus solving #23366.

[vadimcn]

rustc --crate-type=dylib -C link-args=-Wl,-out-implib,library.lib ... will cause linker to create an import library in addition to the .dll.

[steveklabnik]

@retep998 did @vadimcn's suggestion solve this issue?

[steveklabnik]

Closing until proven otherwise.