Rollup merge of #22326 - semarie:compat-cp, r=alexcrichton

 `cp -a` is a GNU extension. Use an alternate combinaison of POSIX options
(`-PRp`) that do nearly the same.

The difference is `-a` will preserve context, links and xattr attributes,
whereas `-p` not. But as we use it only for copy a file, there is no
difference in the current context.

diff --cc mk/docs.mk
Simple merge