[libc] Make RPC malloc implementation return 'nullptr' on alloc failure

Summary:
`malloc` is supposed to return `nullptr` if it fails, not exit with an
error code.

Author: jhuber6

libc/utils/gpu/loader/amdgpu/amdhsa-loader.cpp

@@ -180,7 +180,7 @@ hsa_status_t launch_kernel(hsa_agent_t dev_agent, hsa_executable_t executable,
           if (hsa_status_t err =
                   hsa_amd_memory_pool_allocate(pool, size,
                                                /*flags=*/0, &dev_ptr))
-            handle_error(err);
+            dev_ptr = nullptr;
           hsa_amd_agents_allow_access(1, &dev_agent, nullptr, dev_ptr);
           buffer->data[0] = reinterpret_cast<uintptr_t>(dev_ptr);
         };

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

@@ -198,7 +198,7 @@ CUresult launch_kernel(CUmodule binary, CUstream stream,
           uint64_t size = buffer->data[0];
           CUdeviceptr dev_ptr;
           if (CUresult err = cuMemAllocAsync(&dev_ptr, size, memory_stream))
-            handle_error(err);
+            dev_ptr = 0UL;
 
           // Wait until the memory allocation is complete.
           while (cuStreamQuery(memory_stream) == CUDA_ERROR_NOT_READY)