WIP: generate sysimg.so

DO NOT MERGE

This is an attempt to save and reload the startup image as a shared library, for #260. It gets to repl.c and calls several functions deep into sysimg.so, but segfaults at the first literal pointer (as predicted by Jeff).

- [x] save bitcode
- [x] compile to shared library linked against libjulia
- [x] reload shared library at startup and set pointers
- [ ] replace all uses of literal_pointer_val with... (globals?)
- [ ] ???