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

programs/lib/libc/libc.c

@@ -131,6 +131,7 @@ static inline long _mtk_syscall4(long nr, long a1, long a2, long a3, long a4) {
 #define SYS_SETENVIRON 172
 #define SYS_SPAWN_ENV 173
 #define SYS_ABORT     198
+#define SYS_EXIT_GROUP 199
 
 /* ========================================================================
    errno
@@ -1240,7 +1241,8 @@ int atexit(void (*func)(void)) {
 void exit(int status) {
     for (int i = _atexit_count - 1; i >= 0; i--)
         _atexit_funcs[i]();
-    _mtk_syscall1(SYS_EXIT, (long)status);
+    /* SYS_EXIT from a sibling thread would end only that thread. */
+    _mtk_syscall1(SYS_EXIT_GROUP, (long)status);
     __builtin_unreachable();
 }
 
@@ -4066,7 +4068,7 @@ pid_t waitpid(pid_t pid, int *status, int options) {
 }
 
 void _exit(int status) {
-    _mtk_syscall1(SYS_EXIT, (long)status);
+    _mtk_syscall1(SYS_EXIT_GROUP, (long)status);
     __builtin_unreachable();
 }