rt: Add a Rust callback debugging function

src/rt/rust_builtin.cpp

@@ -654,6 +654,13 @@ rust_dbg_lock_signal(lock_and_signal *lock) {
     lock->signal();
 }
 
+typedef void *(*dbg_callback)(void*);
+
+extern "C" CDECL void *
+rust_dbg_call(dbg_callback *cb, void *data) {
+    return (*cb)(data);
+}
+
 //
 // Local Variables:
 // mode: C++

src/rt/rustrt.def.in

@@ -104,3 +104,4 @@ rust_dbg_lock_lock
 rust_dbg_lock_unlock
 rust_dbg_lock_wait
 rust_dbg_lock_signal
+rust_dbg_call