openocd: fix SPDX tag format for files .c

With the old checkpatch we cannot use the correct format for the
SPDX tags in the file .c, in fact the C99 comments are not allowed
and we had to use the block comment.

With the new checkpatch, let's switch to the correct SPDX format.

Change created automatically through the command:
	sed -i \
	's,^/\* *\(SPDX-License-Identifier: .*[^ ]\) *\*/$,// \1,' \
	$(find src/ contrib/ -name \*.c)

Change-Id: I6da16506baa7af718947562505dd49606d124171
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7153
Tested-by: jenkins

src/target/arc_cmd.c

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0-or-later */
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 /***************************************************************************
  *   Copyright (C) 2013-2015,2019-2020 Synopsys, Inc.                      *