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 2902b3a09e ("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: 2902b3a09e ("Cygwin: drop requirement to build newlib's stdio64")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/include/cygwin/config.h

@@ -49,6 +49,12 @@ extern inline struct _reent *__getreent (void)
 
 #define __FILENAME_MAX__ 4096	/* Keep in sync with PATH_MAX in limits.h. */
 
+/* Unfortunately we defined __LARGE64_FILES until Cygwin 3.3.6, so
+   FILE was based on `struct __sFILE64'.  The name is exposed into
+   userspace and consequentially used in C++ name mangling.  We must
+   redefine __sFILE as __sFILE64 to stay backward compatible. */
+#define __sFILE	__sFILE64
+
 /* The following block of macros is required to build newlib correctly for
    Cygwin.  Changing them in applications has no or not the desired effect.
    Just leave them alone. */