-Fo flag used instead of -o for clang & clang++ on Windows

I should've done more testing on my PR for #480, my object file was stale and I didn't cargo clean to realize there is a further issue. When using -Fo for MSVC cl (and clang-cl) it'll properly write out the object file. 

The new issue I found is that clang is receiving the -Fo arg instead of the -o arg that works like the gnu toolchain flags. I've submitted a PR that fixes this.