[flang] [runtime] Fix build warnings if built with mingw

Check whether `F_OK` et al are defined before redefining them; mingw
headers do define them, so check before providing the windows fallback
defines.

Also check `_WIN32` instead of `WIN32`; this is how it's consistently
done in the rest of llvm. (The former is a compiler builtin define,
while the latter isn't, but it's commonly set by e.g. build systems.)

Differential Revision: https://reviews.llvm.org/D132481