r241432 - Teach mingw toolchain the msys2 mingw-w64 distribution C++ dirs.

[İsmail Dönmez]

[snip]

> P.S: The problem on Cygwin side seems to be that my toolchain is
> missing libgcc_s.a and this seems to be working fine since I build the
> whole toolchain as static. Do you think its reasonable to
> conditionally add -lgcc_s depending on it exists or not?

Honestly though this is non-standard and not worth adding code for.
The situation on Linux is still a regression though :/

Thanks a lot!