Rewrite of build system.

Notable changes:
- We now only build things by default that require
  no dependencies.  So you can build upb even if you
  don't have Lua or Google protobuf installed.
- Checked in a pre-built version of the JIT, so you
  don't need Lua installed at build time to run DynASM.
  It will still notice if you change the .dasc file and
  attempt to re-run DynASM in that case.
- The build system now builds all modules of upb into
  separate libraries, reflecting the modularity that
  is already inherent in upb's design.  This should
  make it easier to trim the fat.
- removed the GDB JIT interface.  I wasn't using it
  much; using a .so is easier and more robust.