Rewrite gen_ext_hhvm and gen_infotabs in C++

Rewrite gen_ext_hhvm and gen_infotabs in C++

This is a near-straight translation of gen_ext_hhvm.php into C++. The
main goal of this change is to break the bootstrap cycle in the build
process: we have to run PHP scripts to build our PHP interpreter. This
isn't so bad for us internally, but in the open-source build, it's
philosophically lame to depend on an external PHP interpreter (i.e.
Zend) to build HHVM. To get around this, we have to check in the
generated ext_hhvm files: no good.

There's nothing particularly sophisticated in this program; it's just a
bunch of grunt work. I took the opportunity to make a few improvements
to the generated code -- there was some int-width confusion, and a bunch of
duplicated code to transfer return values to the right place.