[Sanitizer] Disable arc4random seeding apis on for Non NetBSD platforms.

- arc4random_stir / arc4random_addrandom had been made obsolete (and removed) from FreeBSD 12.

Reviewers: krytarowski

Reviewed By: krytarowski

Differential Revision: https://reviews.llvm.org/D56210

llvm-svn: 350249