[asan] Use MADV_NOCORE for use_madv_dontdump on FreeBSD.

Currently in FreeBSD 12.0-CURRENT with trunk clang+compiler-rt, faulty -fsanitize=address executable hangs at 'urdlck' state.

Ka Ho Ng has verified that by backporting this to llvm 6.0.1, with use_madv_dontdump=1, shadow memory is not dumped.

ASAN_OPTIONS=abort_on_error=1:disable_coredump=0:use_madv_dontdump=1 ./a

Reviewers: dimitry, kcc, dvyukov, emaste, khng300

Subscribers: kubamracek, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D48257

llvm-svn: 336046