[libc] Explicitly pin memory for the client symbol lookup (#73988)

Summary:
Previously, we determined that coarse grained memory cannot be used in
the general case. That removed the buffer used to transfer the memory,
however we still had this lookup. Though we do not access the symbol
directly, it still conflicts with the agents apparently. Pin this as
well.

This resolves the problems @lntue was having with the `libc` GPU build.

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

@@ -464,18 +464,20 @@ int load(int argc, char **argv, char **envp, void *image, size_t size,
           executable, rpc_client_symbol_name, &dev_agent, &rpc_client_sym))
     handle_error(err);
 
-  void *rpc_client_host;
-  if (hsa_status_t err =
-          hsa_amd_memory_pool_allocate(coarsegrained_pool, sizeof(void *),
-                                       /*flags=*/0, &rpc_client_host))
-    handle_error(err);
-
   void *rpc_client_dev;
   if (hsa_status_t err = hsa_executable_symbol_get_info(
           rpc_client_sym, HSA_EXECUTABLE_SYMBOL_INFO_VARIABLE_ADDRESS,
           &rpc_client_dev))
     handle_error(err);
 
+  // Pin some memory we can use to obtain the address of the rpc client.
+  void *rpc_client_storage = nullptr;
+  void *rpc_client_host = nullptr;
+  if (hsa_status_t err =
+          hsa_amd_memory_lock(&rpc_client_storage, sizeof(void *),
+                              /*agents=*/nullptr, 0, &rpc_client_host))
+    handle_error(err);
+
   // Copy the address of the client buffer from the device to the host.
   if (hsa_status_t err = hsa_memcpy(rpc_client_host, host_agent, rpc_client_dev,
                                     dev_agent, sizeof(void *)))
@@ -497,7 +499,7 @@ int load(int argc, char **argv, char **envp, void *image, size_t size,
   if (hsa_status_t err = hsa_amd_memory_unlock(
           const_cast<void *>(rpc_get_client_buffer(device_id))))
     handle_error(err);
-  if (hsa_status_t err = hsa_amd_memory_pool_free(rpc_client_host))
+  if (hsa_status_t err = hsa_amd_memory_unlock(rpc_client_host))
     handle_error(err);
 
   // Obtain the GPU's fixed-frequency clock rate and copy it to the GPU.