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

Reviewed By: ldionne, emaste, dim

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

libcxx/docs/ReleaseNotes.rst

@@ -97,6 +97,11 @@ ABI Changes
   ``_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
 --------------------
 

libcxx/include/__config

@@ -350,7 +350,8 @@
 //      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