[PATCH] libc++: Add support for arc4random() to random_device.

[JF Bastien]

arc4random was broken for a short while on FreeBSD: http://lwn.net/Articles/633805/rss
Not much can be done from libc++, but I'd avoid using it for a while on that platform so we can be reasonably certain they're fixed.

Another idea is having `arc4random` as a fallback on platforms where opening `/dev/urandom` fails and `arc4random` is available. The rationale is that the platforms offers a POSIX-y API but the `/dev/` access was blocked by a sandbox. I'm not sure I like that approach, but I think it's worth mentioning.


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================
Comment at: test/std/numerics/rand/rand.device/ctor.pass.cpp:32
@@ +31,3 @@
+#else
+  return true;
+#endif
----------------
We had a discussion about allowing any string on Windows: I think it's a bad idea. I realize it's implementation dependent, but I'd only allow `/dev/urandom` on new platforms if you don't offer the semantics of `/dev/random`.

http://reviews.llvm.org/D8134

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