Recognize windows-sys signatures as "C" or "system" depending on cfg

[dtolnay]

Closes #676.

This is not an ideal solution because it relies on --cfg=windows_raw_dylib to be passed to backtrace if and only if the same flag is passed to windows-targets. In Cargo this would always be the case and in Buck it's easy to arrange; not sure about different build systems.

I did not come up with a way to write a check that succeeds if super::windows_sys::$name is unsafe extern "C" fn($($argty),*) -> $ret OR unsafe extern "system" fn($($argty),*) -> $ret and fails for all other signatures.

[ChrisDenton]

This will need to exclude x86 because Windows x86 is the only target where the "system" is not the same as "C".

[ChrisDenton]

Thanks!

CI is currently failing for unrelated reasons but the change is fairly straightforward and I've tested locally so I'm happy to merge this to unblock use of raw-dylib.