Cygwin: fstatat: call fstat64 instead of fstat

This fixes a bug on 32-bit Cygwin that was introduced in commit
84252946, "Cygwin: fstatat, fchownat: support the AT_EMPTY_PATH flag".

Add a comment explaining why fstat should not be called.

Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247399.html

winsup/cygwin/release/3.2.0

@@ -42,3 +42,6 @@ Bug Fixes
 
 - Fix return value of sqrtl on negative infinity.
   Addresses: https://cygwin.com/pipermail/cygwin/2020-October/246606.html
+
+- Fix a bug in fstatat(2) on 32 bit that could cause it to return garbage.
+  Addresses: https://cygwin.com/pipermail/cygwin/2021-January/247399.html

winsup/cygwin/syscalls.cc

@@ -1929,6 +1929,9 @@ _fstat64_r (struct _reent *ptr, int fd, struct stat *buf)
 }
 
 #ifdef __i386__
+/* This entry point is retained only to serve old 32 bit applications
+built under Cygwin 1.3.x or earlier.  Newer 32 bit apps are redirected
+to fstat64; see NEW_FUNCTIONS in Makefile.in. */
 extern "C" int
 fstat (int fd, struct stat *buf)
 {
@@ -4852,7 +4855,7 @@ fstatat (int dirfd, const char *__restrict pathname, struct stat *__restrict st,
 	      cwdstuff::cwd_lock.release ();
 	    }
 	  else
-	    return fstat (dirfd, st);
+	    return fstat64 (dirfd, st);
 	}
       path_conv pc (path, ((flags & AT_SYMLINK_NOFOLLOW)
 			   ? PC_SYM_NOFOLLOW : PC_SYM_FOLLOW)