Implement extern decls, which codegen to LLVM as forward-declared
functions, and use these as a hook into calling glibc functions.
We can print to the terminal now! The integration tests can test this
now.
Change-Id: I70af4546b417b888ad9fbb18798db240f77f4e71
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2614
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
