Subzero: Allow Makefile.standalone to build for a single target.

This makes it easier to focus on reducing the translator size.

Enable with e.g.:
  make -f Makefile.standalone MINIMAL=1 SZTARGET=X8664 bloat-sb

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4362
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1787143002 .
5 files changed