Teach clang/NetBSD about additional dependencies for sanitizers

Summary:
Sanitizers on NetBSD require additional linkage:

 - libutil for forkpty(3)
 - libexecinfo for backtrace(3)

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, vitalybuka, kcc

Reviewed By: eugenis

Subscribers: #sanitizers, cfe-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D41054

llvm-svn: 321060