feat: Rust std target + ruststd tests; SYS_EXIT_GROUP; fix mkdir/create under missing parent

programs/rust/rusthello/src/main.rs

@@ -1,4 +1,4 @@
-//! rusthello - Phase 1 Rust smoke test for MontaukOS.
+//! rusthello - `no_std` Rust smoke test for MontaukOS (montauk-rt).
 //!
 //! Exercises alloc collections, formatting, argv, a #[thread_local]
 //! (PT_TLS + per-thread %fs blocks), threads, and the futex mutex.