Annotating CHERI changes

See https://github.com/CTSRD-CHERI/cheribsd/blob/master/tools/tools/cheri-changes/cheri-changes-json.md for the schema.

Scripts to parse the changes: https://github.com/CTSRD-CHERI/cheribsd/tree/master/tools/tools/cheri-changes

Example:

/*
 * CHERI CHANGES START
 * {
 *   "updated": 20180530,
 *   "changes": [
 *     "pointer_size"
 *   ],
 *   "change_comment": "align stack for capabilities",
 *   "hybrid_specific": true
 * }
 * CHERI CHANGES END
 */

Also see https://github.com/CTSRD-CHERI/cheribsd/commit/c24a8afa2b25d59edb7548fe55ff89581947cc17 for a commit with lots of annotations