Cygwin: define FILE as struct __sFILE64, not as __sFILE

Cygwin: define FILE as struct __sFILE64, not as __sFILE

Until Cygwin 3.3.6, we define __LARGE64_FILES unconditionally, so we
were using the type __sFILE64 even for 64 bit.  That was lazy and wrong.
so commit 2902b3a09e0a ("Cygwin: drop requirement to build newlib's
stdio64") tried to fix that.

Unfortunately this patch forgot to take the exposure of the typename
__sFILE64 in userspace into account.  This leads to trouble in C++ due
to name mangling.

Commit 0f376ae22036 tried to fix this by just renaming __sFILE to
__sFILE64 by using a macro.  While __sFILE and __sFILE64 are the same
size, they are not exactly congruent.

To avoid backward compatibility problems, make sure to define FILE
as the  real __sFILE64, and make sure that __sFILE is not defined at
all on Cygwin.

Fixes: 0f376ae22036 ("Cygwin: rename __sFILE to __sFILE64 for backward
compatibility")
Fixes: 2902b3a09e0a ("Cygwin: drop requirement to build newlib's stdio64")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>