mpy-cross: Fix compiler detection for including windows/fmode.c

fmode.c should only be included for builds targetting 'pure' windows, i.e.
msvc or mingw builds but not when using msys or cygwin's gcc (see #2298).
Just checking if the OS is windows and UNAME doesn't have msys stil leaves
the gate open for builds with cygwin's gcc since UNAME there is e.g.
CYGWIN_NT-6.1-WOW.
Fix this by checking for 'mingw' explicitly in the compiler version; both
gcc and clang have the -dumpmachine flag so the check should be ok for
all platforms.