[libc] Test the RPC interface with multiple blocks

The RPC interface can support multiple independent clients. This support
currently only supports many single-thread warps / workgroups
coordinating over a single lock. This patch uses the support added in
the previous patch to test the RPC interface with multiple blocks.

Note that this does not work with multiple threads currently because of
the effect of warps / workgroups executing in lockstep incorrectly. This
will be added later.

Depends on D148485

Reviewed By: lntue, sivachandra

Differential Revision: https://reviews.llvm.org/D148486