[libc] Ensure the RPC server is alwaus used

Summary:
Previous change made us no longer link `exit` by default which implied
the RPC server. This is a required symbol for the loader. This will be
fixed later when I port the loader to LLVMOffload. For now just work
around it to fix the bots.

Author: jhuber6

libc/startup/gpu/amdgpu/start.cpp

@@ -21,6 +21,9 @@ namespace LIBC_NAMESPACE_DECL {
 // FIXME: Factor this out into common logic so we don't need to stub it here.
 void teardown_main_tls() {}
 
+// FIXME: Touch this symbol to force this to be linked in statically.
+volatile void *dummy = &LIBC_NAMESPACE::rpc::client;
+
 DataEnvironment app;
 
 extern "C" uintptr_t __init_array_start[];

libc/startup/gpu/nvptx/start.cpp

@@ -23,6 +23,9 @@ DataEnvironment app;
 // FIXME: Factor this out into common logic so we don't need to stub it here.
 void teardown_main_tls() {}
 
+// FIXME: Touch this symbol to force this to be linked in statically.
+volatile void *dummy = &LIBC_NAMESPACE::rpc::client;
+
 extern "C" {
 // Nvidia's 'nvlink' linker does not provide these symbols. We instead need
 // to manually create them and update the globals in the loader implememtation.