Add a windows_raw_dylib feature

[ChrisDenton]

The standard library recently gained a windows_raw_dylib feature that enables building rust programs without any Windows SDK import libraries (though it still needs C/C++ runtime stuff). However, backtrace doesn't yet support this so at least kernel32 is still required.

I don't have time to work on this atm but if anyone wants to then seeing how the standard library does it might help. Note that std using the unstable macro (instead of macro_rules) so that would likely need to be different in backtrace-rs unless gating on backtrace_in_libstd.