Please add `cc.exe`

[Curculigo]

It's only a copy of gcc.exe, but it should better be there. Some build systems still hardcode CC and CXX to cc and c++ respectively. I found that you already have c++.exe. It's better to just add cc.exe instead of having the users to set CC variable manually.

I'm using your GCC in MSYS2. I have the Cygwin GCC installed (because I sometimes need to test some POSIX programs). If there is no cc.exe and CC is not set to gcc manually, /usr/bin/cc which is the Cygwin GCC will be picked.

Note: Not only cc.exe, Cygwin GCC will also install c89.exe, c99.exe, and c11.exe. If you can please add them, too. Thank you.