msun: Add copyright notices

These files were copied from MUSL. Add the standard copyright notice and
SPDX-License-Identifier: MIT consistent with our new draft license
policy. It reads word for word the same as the MIT license on the SPDX
web site. Add a pointer to the MUSL COPYIRGHT file which contains a list
of all authors of MUSL.

Sponsored by:		Netflix
Noticed by:		Steve Kargl

lib/msun/src/s_scalbn.c

@@ -1,3 +1,11 @@
+/*
+ * Copyright (c) 2005-2020 Rich Felker, et al.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Please see https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+ * for all contributors to musl.
+ */
 #include <float.h>
 #include <math.h>
 #include <stdint.h>

lib/msun/src/s_scalbnf.c

@@ -1,3 +1,11 @@
+/*
+ * Copyright (c) 2005-2020 Rich Felker, et al.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Please see https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+ * for all contributors to musl.
+ */
 #include <math.h>
 #include <stdint.h>
 

lib/msun/src/s_scalbnl.c

@@ -1,3 +1,11 @@
+/*
+ * Copyright (c) 2005-2020 Rich Felker, et al.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Please see https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+ * for all contributors to musl.
+ */
 #include <math.h>
 #include <float.h>
 #include "math_private.h"