[libc] Fix llvm-gpu-loader passing uninitialized device memory (llvm#186804)

Summary:
The return value was not zeroed, this was accidentally dropped when we
did the port and it's zero "almost always" so I didn't notice. Hopefully
this makes the test suite no longer flaky.

Author: jhuber6

llvm/tools/llvm-gpu-loader/llvm-gpu-loader.cpp

@@ -268,7 +268,9 @@ int main(int argc, const char **argv, const char **envp) {
   void *DevEnvp = copyEnvironment(envp, Device);
 
   void *DevRet;
+  int Zero = 0;
   OFFLOAD_ERR(olMemAlloc(Device, OL_ALLOC_TYPE_DEVICE, sizeof(int), &DevRet));
+  OFFLOAD_ERR(olMemcpy(Queue, DevRet, Device, &Zero, Host, sizeof(int)));
 
   ol_kernel_launch_size_args_t BeginLaunch{1, {1, 1, 1}, {1, 1, 1}, 0};
   BeginArgs BeginArgs = {DevArgc, DevArgv, DevEnvp};
@@ -291,6 +293,7 @@ int main(int argc, const char **argv, const char **envp) {
   OFFLOAD_ERR(olMemcpy(Queue, &Ret, Host, DevRet, Device, sizeof(int)));
   OFFLOAD_ERR(olSyncQueue(Queue));
 
+  OFFLOAD_ERR(olMemFree(DevRet));
   OFFLOAD_ERR(olMemFree(DevArgv));
   OFFLOAD_ERR(olMemFree(DevEnvp));
   OFFLOAD_ERR(olDestroyQueue(Queue));