🆕 libc新增_start函数，实现main的返回值的捕获

user/libs/libc/sysdeps/x86_64/elf/start.c

@@ -0,0 +1,10 @@
+
+extern int main(int, char**);
+#include<libc/stdio.h>
+void _start(int argc, char** argv)
+{
+    printf("before main\n");
+    int retval = main(argc, argv);
+    printf("before exit, code=%d\n", retval);
+    exit(retval);
+}