toolchain/zephyr: invoke ar with -D for deterministic .a files

Quoting GNU ar man/info page:

  'D'
     Operate in _deterministic_ mode.  When adding files and the archive
     index use zero for UIDs, GIDs, timestamps, and use consistent file
     modes for all files.  When this option is used, if 'ar' is used
     with identical options and identical input files, multiple runs
     will create identical output files regardless of the input files'
     owners, groups, file modes, or modification times.

     If 'binutils' was configured with
     '--enable-deterministic-archives', then this mode is on by default.
     It can be disabled with the 'U' modifier, below.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>