This change now supports building subzero as part of the LLVM build (instead
of in a separate build step). It is modeled on clang's Makefiles.

The existing Makefile has been renamed and can still be used manually, e.g.
Make -f Makefile.standalone

It does not yet support running tests, just building.

R=stichnot@chromium.org, jvoung@chromium.org
BUG=

Review URL: https://codereview.chromium.org/293983007