Add support for creating COFF archives

[dpaoliello]

Currently COFF is not supported:
https://github.com/rust-lang/ar_archive_writer/blob/master/src/archive_writer.rs#L42

However, we need to build rlibs as COFF for MSVC when using LLVM as a backend so that LLVM generates symbol tables: rust-lang/rust#123467

[bjorn3]

LLVM added support for them last year. Ar_archive_writer will need to adopt all changes since the commit it was derived from.