Cygwin: rename __sFILE to __sFILE64 for backward compatibility

Cygwin: rename __sFILE to __sFILE64 for backward compatibility

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.

Fix this by redefining __sFILE to __sFILE64.  The type name is very much
internal, so it doesn't really matter, except for the fact that it needs
to stay backward compatible so as not to break building against C++ libs
built under older versions of Cygwin.

Fixes: 2902b3a09e0a ("Cygwin: drop requirement to build newlib's stdio64")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>