bugfix: use OS default permissions for mkdirp when mode is not provided (#9604)
bugfix: use OS default permissions for mkdirp when mode is not provided (#9604)

- #8773 made the default mode 0o777, which is what's documented but
   mkdirp actually takes the OS default or umask by default

- revert to the Python default by default, and only set the mode when
  asked explicitly.
LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack