Enable getentropy for FreeBSD 12

As for Linux with its getrandom's syscall, giving the possibility to fill buffer with native call for good quality but falling back to /dev/urandom in worst case similarly.

Reviewers: vitalybuka, krytarowski

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D48804

llvm-svn: 339318