Reinstantiate Cygwin function called `__getreent'

This partially reverts commit 10a30e7 as far as the Cygwin version of
the __getreent function is concerned.  Remove _COMPILING_NEWLIB guard
only allowing to use __getreent inline function when building newlib,
since we wan to use it in Cygwin as well.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Stefan Assmann <sassmann@redhat.com>

winsup/cygwin/thread.cc

@@ -80,6 +80,13 @@ pthread_mutex::no_owner()
     return res;
 }
 
+#undef __getreent
+extern "C" struct _reent *
+__getreent ()
+{
+  return &_my_tls.local_clib;
+}
+
 extern "C" void
 __cygwin_lock_init (_LOCK_T *lock)
 {