Common: Add Random utilities

This makes it easier to generate random numbers or fill a buffer with
random data in a cryptographically secure way.

This also replaces existing usages of RNG functions in the codebase:

* <random> is pretty hard to use correctly, and std::random_device does
  not give enough guarantees about its results (it's
  implementation-defined, non cryptographically secure and could be
  deterministic on some platforms).
  Doing things correctly is error prone and verbose.

* rand() is terrible and should not be used especially in crypto code.

Source/Core/Common/Random.h

@@ -0,0 +1,15 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <cstddef>
+
+#include "Common/CommonTypes.h"
+
+namespace Common::Random
+{
+/// Fill `buffer` with random bytes using a cryptographically secure pseudo-random number generator.
+void Generate(void* buffer, std::size_t size);
+}  // namespace Common::Random