[libcxx] random_device, use arc4random() on FreeBSD, NetBSD and DragonFlyBSD

[brad]

Summary

With OpenBSD and now macOS using arc4random() for the random device, might as well look at the other BSD's that have arc4random().

[brad]

@emaste ? @krytarowski ?

[ldionne]

LGTM if the contact points for those OSes are happy.

[ldionne]

I'll note that this is a behavior change though. When I made the change on Apple, I added this release note:

On Apple platforms, std::random_device is now implemented on top of arc4random() instead of reading from /dev/urandom. Any implementation-defined token used when constructing a std::random_device will now be ignored instead of interpreted as a file to read entropy from.

Please add a similar release note here.

[emaste]

Sounds good to me for FreeBSD (w/ the release note as mentioned by @ldionne) arc4random is ChaCha20 in all supported FreeBSD versions.

[dim]

Sure, LGTM.

[brad]

Add release note.

[brad]

@mgorny ? @krytarowski ?

libcxx/docs/ReleaseNotes.rst

 ABI Changes
 -----------
 
 - The ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` macro controlling whether we use an
   emulation for ``std::nullptr_t`` in C++03 mode has been removed. After this change,
   ``_LIBCPP_ABI_USE_CXX03_NULLPTR_EMULATION`` will not be honoured anymore and there
   will be no way to opt back into the C++03 emulation of ``std::nullptr_t``.
 
+- On FreeBSD, NetBSD and DragonFlyBSD, ``std::random_device`` is now implemented on top of
+  ``arc4random()`` instead of reading from ``/dev/urandom``. Any implementation-defined
+  token used when constructing a ``std::random_device`` will now be ignored instead of
+  interpreted as a file to read entropy from.
+
 Build System Changes
 --------------------
 
 - Support for standalone builds have been entirely removed from libc++, libc++abi and
   libunwind. Please use :ref:`these instructions <build instructions>` for building
   libc++, libc++abi and/or libunwind.
 
 - The ``{LIBCXX,LIBCXXABI,LIBUNWIND}_TARGET_TRIPLE``, ``{LIBCXX,LIBCXXABI,LIBUNWIND}_SYSROOT`` and

libcxx/include/__config

 //      uses the NaCL syscall `nacl_secure_random_init()` to get entropy.
 //      When this option is used, the token passed to `std::random_device`'s
 //      constructor *must* be "/dev/urandom" -- anything else is an error.
 //
 // _LIBCPP_USING_WIN32_RANDOM
 //      Use rand_s(), for use on Windows.
 //      When this option is used, the token passed to `std::random_device`'s
 //      constructor *must* be "/dev/urandom" -- anything else is an error.
-#if defined(__OpenBSD__) || defined(__APPLE__)
+#if defined(__APPLE__) || defined(__FreeBSD__) || defined(__NetBSD__) || \
+    defined(__OpenBSD__) || defined(__DragonFly__)
 #  define _LIBCPP_USING_ARC4_RANDOM
 #elif defined(__wasi__)
 #  define _LIBCPP_USING_GETENTROPY
 #elif defined(__Fuchsia__)
 #  define _LIBCPP_USING_FUCHSIA_CPRNG
 #elif defined(__native_client__)
 #  define _LIBCPP_USING_NACL_RANDOM
 #elif defined(_LIBCPP_WIN32API)