sysconf: add _SC_THREAD_THREADS_MAX response

fix ltp_pthread_create_s_c1 test get wrong PTHREAD_THREADS_MAX.

Signed-off-by: yangyalei <yangyalei@xiaomi.com>

libs/libc/unistd/lib_sysconf.c

@@ -249,6 +249,11 @@ long sysconf(int name)
       case _SC_THREAD_STACK_MIN:
         return CONFIG_PTHREAD_STACK_MIN;
 
+      /* threads limit to tcb_s->group->tg_nmembers(unint_8) */
+
+      case _SC_THREAD_THREADS_MAX:
+        return UINT8_MAX;
+
       default:
 #if 0 /* Assume valid but not implemented for the time being */
         errcode = EINVAL;