[libc] Add a test to directly stimulate the RPC interface

Currently, the RPC interface with the loader is only tested if the other
tests fail. This test adds a direct test that runs a simple integer
increment over the RPC handshake 10000 times.

Depends on https://reviews.llvm.org/D148288

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D148342