[libcxx] random_device, specify optimal entropy properties for all OS's using arc4random()

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D122522

(cherry picked from commit 6e2c6c9)

Author: brad0

libcxx/src/random.cpp

@@ -210,7 +210,7 @@ random_device::entropy() const noexcept
     return std::numeric_limits<result_type>::digits;
 
   return ent;
-#elif defined(__OpenBSD__) || defined(_LIBCPP_USING_FUCHSIA_CPRNG)
+#elif defined(_LIBCPP_USING_ARC4_RANDOM) || defined(_LIBCPP_USING_FUCHSIA_CPRNG)
   return std::numeric_limits<result_type>::digits;
 #else
   return 0;