An assembly-free entrypoint?

entrypoint.c

#if __amd64
#define SYS_write 1
#define SYS_exit  60
__attribute__((force_align_arg_pointer)) void _start(void);
static long syscall3(long n, long a, long b, long c)
{
    long r;
    __asm volatile (
        "syscall"
        : "=a"(r)
        : "a"(n), "D"(a), "S"(b), "d"(c)
        : "rcx", "r11", "memory"
    );
    return r;
}
#endif

#if __aarch64__
#define SYS_write 64
#define SYS_exit  93
static long syscall3(long n, long a, long b, long c)
{
    register long x0 __asm("x0") = a;
    register long x1 __asm("x1") = b;
    register long x2 __asm("x2") = c;
    register long x8 __asm("x8") = n;
    __asm volatile (
        "svc #0"
        : "=r"(x0)
        : "r"(x8), "r"(x0), "r"(x1), "r"(x2)
        : "memory"
    );
    return x0;
}
#endif

void _start(void)
{
    long *argc_ptr = ((long *)__builtin_frame_address(0)) + 1;
    long argc = *argc_ptr;
    char **argv = (char **)(argc_ptr + 1);
    char **envp = (char **)(argc_ptr + 1 + argc + 1);

    for (int i = 0; i < argc; i++) {
        int len = 0;
        for (; argv[i][len]; len++);
        syscall3(SYS_write, 1, (long)argv[i], len);
        syscall3(SYS_write, 1, (long)"\n", 1);
    }

    for (char **e = envp; *e; e++) {
        int len = 0;
        for (; (*e)[len]; len++);
        syscall3(SYS_write, 1, (long)*e, len);
        syscall3(SYS_write, 1, (long)"\n", 1);
    }
    syscall3(SYS_exit, 0, 0, 0);
    __builtin_unreachable();
}