Do not link to caml_startup in ocaml_runtime
Summary: It looks like ocaml_binary rules--at least in buck1 `mode/dev-cov`--do not define a symbol named `caml_startup`, so we get a link error in that mode when the `ocaml_runtime` crate (which defines a wrapper for `caml_startup`) is reachable. Only the `hh_distc` worker (which is not an ocaml_binary) actually needs to invoke `caml_startup`, so move the `caml_startup` wrapper into a new crate which is not reachable from any ocaml_binary rules.
Reviewed By: shayne-fletcher
Differential Revision:
D43136131
fbshipit-source-id:
0dd68a30649d57afa03f6b7e9bea3420e55cb250