[libc][obvious] Actually return the value from `malloc` for NVPTX

[jhuber6]

Summary

Switching to this interface we neglected to actually write the output
from the malloc call to the RPC buffer. Fix this so the tests pass
again.

libc/utils/gpu/loader/nvptx/Loader.cpp

rpc_register_callback(
           uint64_t size = buffer->data[0];
           CUdeviceptr dev_ptr;
           if (CUresult err = cuMemAllocAsync(&dev_ptr, size, memory_stream))
             handle_error(err);
 
           // Wait until the memory allocation is complete.
           while (cuStreamQuery(memory_stream) == CUDA_ERROR_NOT_READY)
             ;
+          buffer->data[0] = static_cast<uintptr_t>(dev_ptr);
         };
         rpc_recv_and_send(port, malloc_handler, data);
       },
       &memory_stream);
   rpc_register_callback(
       device_id, RPC_FREE,
       [](rpc_port_t port, void *data) {
         auto free_handler = [](rpc_buffer_t *buffer, void *data) {