cmSystemTools: Improve CreateLink and CreateSymlink error codes

In commit 7f890539 (cmSystemTools: Return KWSys Status from CreateLink
and CreateSymlink, 2021-04-15) we just took the `-err` from libuv and
treated it as a POSIX error.  This is accurate on POSIX, but on Windows
does not match the POSIX error codes.

Use `uv_fs_get_system_error` to get the actual system error code.
This requires libuv 1.38 or higher.  Require that for Windows, but
fall back to the previous approach on POSIX.