[libc] Make LIBC_EXIT RPC code use quick exit

Summary:
This RPC call does the final exiting. The callbacks were handled on the
GPU side and this is only 'valid' in the pretend mode where we treat the
GPU like a CPU program. Doing this keeps us from crashing and burning
if people continue using the program while this is running as `exit`
would tear down the offloading library in memory and lead to segfaults.
This just drops everything where it is and lets the process manager
clean it up for us.

Author: jhuber6

libc/src/__support/RPC/rpc_server.h

@@ -395,7 +395,9 @@ LIBC_INLINE static rpc::Status handle_port_impl(rpc::Server::Port &port) {
     port.recv([](rpc::Buffer *buffer, uint32_t) {
       int status = 0;
       __builtin_memcpy(&status, buffer->data, sizeof(int));
-      exit(status);
+      // We want a quick exit to avoid conflicts with offloading library
+      // teardowns when called from the GPU.
+      quick_exit(status);
     });
     break;
   }