architecture: make it easier to get a Spec for the default arch
architecture: make it easier to get a Spec for the default arch

- [x] Make it easier to get a `Spec` with a proper `ArchSpec` from an
      `Arch` object via new `Arch.to_spec()` method.

- [x] Pull `spack.architecture.default_arch()` out of
      `spack.architecture.sys_type()` so we can get an `Arch` instead of
      a string.

iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack