FreeBSD needs also execinfo (in sanitizers)

Summary: As NetBSD, FreeBSD needs execinfo for backtrace's matters.

Patch by: David CARLIER.

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: emaste, krytarowski, cfe-commits

Differential Revision: https://reviews.llvm.org/D42467

llvm-svn: 323387