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 .