[libc] Export GPU extensions to `libc` for external use

The GPU port of the LLVM C library needs to export a few extensions to
the interface such that users can interface with it. This patch adds the
necessary logic to define a GPU extension. Currently, this only exports
a `rpc_reset_client` function. This allows us to use the server in
D147054 to set up the RPC interface outside of `libc`.

Depends on https://reviews.llvm.org/D147054

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D152283