Use this source use on Fuchsia where this is the oficially way
to obtain randomness. This could be also used on other platforms
that already support getentropy such as *BSD or Linux.
Differential Revision: https://reviews.llvm.org/D40319
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@319523 91177308-0d34-0410-b5e6-96231b3b80d8