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

[brad]

Summary

After noticing D116045, fix the function to take into consideration other OS's using arc4random().

[ldionne]

Thanks for noticing! We should also cherry-pick this to release/14.x, let me know if you want me to do it.

[brad]

In D122522#3412147, @ldionne wrote:

Thanks for noticing! We should also cherry-pick this to release/14.x, let me know if you want me to do it.

Done. 54605.

[ldionne]

In D122522#3413039, @brad wrote:

In D122522#3412147, @ldionne wrote:

Thanks for noticing! We should also cherry-pick this to release/14.x, let me know if you want me to do it.

Done. 54605.

Thank you!

libcxx/src/random.cpp

#if defined(_LIBCPP_USING_DEV_RANDOM) && defined(RNDGETENTCNT)
 
   if (ent < 0)
     return 0;
 
   if (ent > std::numeric_limits<result_type>::digits)
     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;
 #endif
 }
 
 _LIBCPP_END_NAMESPACE_STD