Add back openat() wrapper

It's originating from libc in a way that we can't wrap. Here's the impl, not tested obviously

int openat(int dirfd, const char* pathname, int flags, ...)
{
    void* ptr = prep_syscall(NO_DESCHED);
    int mode = 0;
    long ret;

    if (O_CREAT & flags) {
        va_list mode_arg;
        va_start(mode_arg, flags);
        mode = va_arg(mode_arg, int);
        va_end(mode_arg);
    }
    if (!can_buffer_syscall(ptr)) {
        return syscall(SYS_openat, dirfd, pathname, flags, mode);
    }
    ret = untraced_syscall4(SYS_openat, dirfd, pathname, flags, mode);
    return commit_syscall(SYS_openat, ptr, ret, NO_DESCHED);
}

No file or test is named. Start by locating the existing syscall-wrapper entry points for openat and reviewing the supplied implementation, including its variadic mode handling and SYS_openat calls. Done means openat is wrapped again and its behavior is covered by the relevant existing tests.