[libc] Make the RPC server target always available

This patch makes sure that we always build the RPC server. The proposed
used for this is to begin integrating this server implementation into
`libomptarget`. That requires that we build this server ahead of time
when using a `LLVM_ENABLE_PROJECTS` build. Make a few tweaks to ensure
that the GCC compiler which may be used for this build doesn't complain.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D154105