[libc] Replace the `PRINT_TO_STDERR` opcode for RPC printing.

[jhuber6]

Summary

A previous patch added general support for printing via the RPC
interface. we should consolidate this functionality and get rid of the
old opcode that was used for simple testing.

libc/src/__support/OSUtil/gpu/io.cpp

 
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/RPC/rpc_client.h"
 #include "src/string/memory_utils/memcpy_implementations.h"
 
 namespace __llvm_libc {
 
 void write_to_stderr(cpp::string_view msg) {
-  rpc::Client::Port port = rpc::client.open<rpc::PRINT_TO_STDERR>();
+  rpc::Client::Port port = rpc::client.open<rpc::WRITE_TO_STDERR>();
   port.send_n(msg.data(), msg.size());
   port.recv([](rpc::Buffer *) { /* void */ });
   port.close();
 }
 
 } // namespace __llvm_libc

libc/src/__support/RPC/rpc.h

 enum Opcode : uint16_t {
   NOOP = 0,
   EXIT = 1,
   WRITE_TO_STDOUT = 2,
   WRITE_TO_STDERR = 3,
   WRITE_TO_STREAM = 4,
   MALLOC = 5,
   FREE = 6,
-  PRINT_TO_STDERR = 7,
-  TEST_INCREMENT = 8,
-  TEST_INTERFACE = 9,
-  TEST_STREAM = 10,
+  TEST_INCREMENT = 7,
+  TEST_INTERFACE = 8,
+  TEST_STREAM = 9,
 };
 
 /// A fixed size channel used to communicate between the RPC client and server.
 struct Buffer {
   uint64_t data[8];
 };
 static_assert(sizeof(Buffer) == 64, "Buffer size mismatch");
 

libc/utils/gpu/loader/Server.h

case rpc::Opcode::WRITE_TO_STDOUT: {
         reinterpret_cast<int *>(buffer->data)[0] = ret >= 0 ? sizes[id] : ret;
       });
       for (uint64_t i = 0; i < rpc::MAX_LANE_SIZE; ++i) {
         if (strs[i])
           delete[] reinterpret_cast<uint8_t *>(strs[i]);
       }
       break;
     }
-    case rpc::Opcode::PRINT_TO_STDERR: {
-      uint64_t sizes[rpc::MAX_LANE_SIZE] = {0};
-      void *strs[rpc::MAX_LANE_SIZE] = {nullptr};
-      port->recv_n(strs, sizes, [&](uint64_t size) { return new char[size]; });
-      port->send([](rpc::Buffer *) { /* void */ });
-      for (uint64_t i = 0; i < rpc::MAX_LANE_SIZE; ++i) {
-        if (strs[i]) {
-          fwrite(strs[i], sizes[i], 1, stderr);
-          delete[] reinterpret_cast<uint8_t *>(strs[i]);
-        }
-      }
-      break;
-    }
     case rpc::Opcode::EXIT: {
       port->recv([](rpc::Buffer *buffer) {
         exit(reinterpret_cast<uint32_t *>(buffer->data)[0]);
       });
       break;
     }
     case rpc::Opcode::MALLOC: {
       port->recv_and_send([&](rpc::Buffer *buffer) {