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

programs/include/libc/montauk.h

@@ -222,6 +222,7 @@ extern "C" {
 #define MTK_SYS_FUTEX_WAIT          196
 #define MTK_SYS_FUTEX_WAKE          197
 #define MTK_SYS_ABORT               198
+#define MTK_SYS_EXIT_GROUP          199
 /* @SYSCALLS-END */
 
 /* Syscall status codes (montauk/statuscodes.h for C++). */