add Arm to the copyright file

Several math functions are now from the ARM optimized-routines repo
licensed under standard MIT terms and copyrighted by Arm Limited,
so mention this in the COPYRIGHT too.

COPYRIGHT

@@ -122,7 +122,8 @@ Copyright © 1993,2004 Sun Microsystems or
 Copyright © 2003-2011 David Schultz or
 Copyright © 2003-2009 Steven G. Kargl or
 Copyright © 2003-2009 Bruce D. Evans or
-Copyright © 2008 Stephen L. Moshier
+Copyright © 2008 Stephen L. Moshier or
+Copyright © 2017-2018 Arm Limited
 and labelled as such in comments in the individual source files. All
 have been licensed under extremely permissive terms.