Implement -fexternal-interpreter via static linking

An interesting idea from @hvr. When using -fexternal-interpreter, we can statically link the required packages and object files into a new iserv binary, and use that as our external interpreter.

This would mean we could support TH and even limited GHCi without needing a dynamic linker of any kind, which is extremely cool. Apparently this would be useful for AIX where doing dynamic linking is hard.