MinGW can't symbolize system libraries

[alexcrichton]

Running cargo run --example backtrace the last two frames are <unknown>. They come from kernel32.dll and ntdll.dll I believe, and they should have symbols (they do on MSVC at least). I'm not sure if we're missing the symbol table on these DLL files or misinterpreting them, but either way I'm pretty certain the symbol is somewhere on the filesystem so we should be able to find it!