[libc] Propagate entrypoint deps to downstream targets.

Deps are recrusively evaluated at the place they are needed. With this
change, one does not have to list recursive deps of entrypoints when
listing test targets. One will still have to explicitly list all
entrypoint objects when setting up an "add_entrypoint_library" target.

Reviewers: abrachet

Differential Revision: https://reviews.llvm.org/D78537