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

Reviewed By: ldionne, emaste, dim

Differential Revision: https://reviews.llvm.org/D122628

NOKEYCHECK=True
GitOrigin-RevId: d13f5023891c92fc79818e02a838f913bf3e452b
diff --git a/docs/ReleaseNotes.rst b/docs/ReleaseNotes.rst
index ddb785b..0445b7b 100644
--- a/docs/ReleaseNotes.rst
+++ b/docs/ReleaseNotes.rst
@@ -97,6 +97,11 @@
   ``_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
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