update copyright file for recent contributions

COPYRIGHT

@@ -65,12 +65,16 @@ were released into the public domain; others are licensed under the
 standard MIT license terms at the top of this file. See individual
 files for their copyright status.
 
-The mips port was originally written by Richard Pennington for use in
-the ellcc project. The original code was adapted by Rich Felker for
-build system and code conventions during upstream integration. It is
-licensed under the standard MIT terms.
+The mips and microblaze ports were originally written by Richard
+Pennington for use in the ellcc project. The original code was adapted
+by Rich Felker for build system and code conventions during upstream
+integration. It is licensed under the standard MIT terms.
 
-All files which have no copyright comments are original works
+The powerpc port was also originally written by Richard Pennington,
+and later supplemented and integrated by John Spencer. It is licensed
+under the standard MIT terms.
+
+All other files which have no copyright comments are original works
 Copyright © 2005-2012 Rich Felker, the main author of this library.
 The decision to exclude such comments is intentional, as it should be
 possible to carry around the complete source code on tiny storage