[libc] Fix RPC server global after mass replace of __llvm_libc

Summary:
This variable needs a reserved name starting with `__`. It was
mistakenly changed with a mass replace. It happened to work because the
tests still picked up the associated symbol, but it just became a bad
name because it's not reserved anymore.

Author: jhuber6

libc/src/__support/RPC/rpc_client.cpp

@@ -18,7 +18,7 @@ Client client;
 /// Externally visible symbol to signify the usage of an RPC client to
 /// whomever needs to run the server as well as provide a way to initialize
 /// the client with a copy..
-extern "C" [[gnu::visibility("protected")]] void *LIBC_NAMESPACE_rpc_client =
+extern "C" [[gnu::visibility("protected")]] const void *__llvm_libc_rpc_client =
     &client;
 
 } // namespace rpc

libc/utils/gpu/server/rpc_server.h

@@ -22,7 +22,7 @@ const uint64_t RPC_MAXIMUM_PORT_COUNT = 512;
 
 /// The symbol name associated with the client for use with the LLVM C library
 /// implementation.
-const char *const rpc_client_symbol_name = "LIBC_NAMESPACE_rpc_client";
+const char *const rpc_client_symbol_name = "__llvm_libc_rpc_client";
 
 /// status codes.
 typedef enum {