Cygwin: fix mknod (64-bit only)

The current definition of mknod in syscalls.cc has a third argument of
type __dev16_t instead of dev_t.  Fix this on 64-bit Cygwin by making
the existing mknod 32-bit only and then exporting mknod as an alias
for mknod32.  (No fix is needed on 32-bit because mknod is redirected
to mknod32 via NEW_FUNCTIONS in Makefile.am.)

Addresses: https://cygwin.com/pipermail/cygwin-developers/2022-May/012589.html

winsup/cygwin/release/3.3.6

@@ -7,3 +7,6 @@ Bug Fixes
 - Fix killpg failing because the exec'ing as well as the exec'ed
   process are not in the pidlist for a brief moment.
   Addresses: https://cygwin.com/pipermail/cygwin/2022-May/251479.html
+
+- Fix mknod (64-bit only), whose definition didn't match its prototype.
+  Addresses: https://cygwin.com/pipermail/cygwin-developers/2022-May/012589.html

winsup/cygwin/syscalls.cc

@@ -3490,11 +3490,15 @@ mknod32 (const char *path, mode_t mode, dev_t dev)
   return -1;
 }
 
+#ifdef __i386__
 extern "C" int
 mknod (const char *_path, mode_t mode, __dev16_t dev)
 {
   return mknod32 (_path, mode, (dev_t) dev);
 }
+#else
+EXPORT_ALIAS (mknod32, mknod)
+#endif
 
 extern "C" int
 mkfifo (const char *path, mode_t mode)