Subzero: Provide "make sb" target for sandboxed pnacl-sz.

Instead of "./pnacl-sz <args>", one can run:

  ../../../run.py ./pnacl-sz.x8632.nexe <args>
or
  ../../../run.py ./pnacl-sz.x8664.nexe <args>

Hopefully the translator performance characteristics are close to that of the browser hookup.

Adds sb builds to two of the presubmit configurations.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1738633002 .