rt: Add a logging function for strings (only).

This will be the shape-free replacement for upcall_log_type.

src/rt/rust_builtin.cpp

@@ -950,6 +950,13 @@ rust_call_tydesc_glue(void *root, size_t *tydesc, size_t glue_index) {
         glue_fn(0, 0, 0, root);
 }
 
+// Don't run on the Rust stack!
+extern "C" void
+rust_log_str(uint32_t level, const char *str) {
+    rust_task *task = rust_get_current_task();
+    task->sched_loop->get_log().log(task, level, "%s", str);
+}
+
 //
 // Local Variables:
 // mode: C++