libgcc: Update 'gthr-mcf.h' to include a dedicated header for libobjc

'libobjc/thr.c' includes 'gthr.h'. While all the other gthread headers
have `#ifdef _LIBOBJC` checks, and provide a different set of inline
functions, I think having one header provide two completely unrelated
set of APIs is unsatisfactory, complicates maintenance, and hinders
further development.

This commit references a new header for libobjc, and adds a copyright
notice, as in other headers.

libgcc/ChangeLog:
	* config/i386/gthr-mcf.h: Include 'gthr_libobjc.h' when building
	libobjc, instead of 'gthr.h'

libgcc/config/i386/gthr-mcf.h

@@ -1 +1,36 @@
+/* Threads compatibility routines for libgcc and libobjc.  */
+/* Copyright (C) 2022 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+Under Section 7 of GPL version 3, you are granted additional
+permissions described in the GCC Runtime Library Exception, version
+3.1, as published by the Free Software Foundation.
+
+You should have received a copy of the GNU General Public License and
+a copy of the GCC Runtime Library Exception along with this program;
+see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+<http://www.gnu.org/licenses/>.  */
+
+#ifdef _LIBOBJC
+
+/* libobjc references some internal structures and requires a
+ * dedicated set of functions.  */
+#include <mcfgthread/gthr_libobjc.h>
+
+#else  /* _LIBOBJC  */
+
+/* This is for libgcc and libstdc++.  */
 #include <mcfgthread/gthr.h>
+
+#endif  /* _LIBOBJC  */