Request init/fini array on FreeBSD 12 and later

Summary:

It seems a bad idea to change the default in the middle of a release
branch due to possible changes in global ctor / dtor ordering between
.ctors and .init_array. With FreeBSD 11.0's release imminent lets
change the default now for FreeBSD 12 (the current development
stream) and later.

FreeBSD rtld has supported .init_array / .fini_array for many years.
As of Jan 1 2017 all supported FreeBSD releases and branches will
have support.

Reviewers: dim, brooks, arichardson

Reviewed By: dim, brooks, arichardson

Subscribers: bsdjhb, krytarowski, emaste, cfe-commits

Differential Revision: https://reviews.llvm.org/D24867