GetRandom / OpenBSD, using getentropy to get high quality randomness

Summary:
- getentropy presence since late 2014, safe to use.
- guarantees to delivers good random data up to 256 bytes.
- fall back to /dev/urandom as long the buffer is correct.

Patch by David CARLIER

Reviewers: kubamracek, vitalybuka

Reviewed By: vitalybuka

Subscribers: cryptoad, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44866

llvm-svn: 329633