[Bug libstdc++/67578] std::random_device::entropy() always returns 0

[ryxi at stu dot xidian.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67578

--- Comment #2 from Xi Ruoyao <ryxi at stu dot xidian.edu.cn> ---
(In reply to Xi Ruoyao from comment #1)

> Or maybe we should keep the ABI unchanged and put all the implementation
> in the header random.h?

No.  It would then pollute the global namespace with sys/ioctl.h and
linux/random.h.  Worse than an ABI minor ("acceptable" in libstdc++ ABI doc)
change.