00_start.c is too hacked on x86_64. it'll work but you're getting a less efficient binary since gcc has to assume _start is called like a normal C function (e.g. it creates a preamble). you should just implement it in assembly.

__init() itself also needs some work. the argument list is weird, linux pushes all of argc, argv, and environ on the stack. why special case argc? also your method of deriving argv and environ from the function argument's address is extremely brittle, and i don't think it actually works on x86_64 (if it does, that's really lucky). you aren't calculating envp using argc, so it's probably wrong. you could get more efficient code from using __attribute__((noreturn)). this would be better:

    /* called from _start */
    void __init(void *initial_stack) __attribute__((noreturn));    
    void __init(void *initial_stack) {
        int argc = *(int *) initial_stack;
        char **argv = ((char **) initial_stack) + 1;
        /* assert(!argv[argc]); */
        char **envp = __environ = argv + argc + 1;

        _exit(main(argc, argv, envp));
    }