[libc] Add RPC helpers for dispatching functions to the host (#179085)

Summary:
The RPC interface is useful for forwarding functions. This PR adds
helper functions for doing a completely bare forwarding of a function
from the client to the server. This is intended to facilitate
heterogenous libraries that implement host functions on the GPU (like
MPI or Fortran).
GitOrigin-RevId: 6d6feb765517cd20667366b750e9eaad6daa818a