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

kernel/src/Api/Syscall.cpp

@@ -403,6 +403,9 @@ namespace montauk::abi {
             case SYS_ABORT:
                 Sys_Abort();
                 return Success;
+            case SYS_EXIT_GROUP:
+                Sys_ExitGroup((int)frame->arg1);
+                return Success;
             case SYS_SETSESSION:
                 return (int64_t)Sys_SetSession();
             case SYS_KILLSESSION: