Link NetBSD with execinfo (CMAKE build)

Summary: FreeBSD and NetBSD share the same library execinfo.

Reviewers: joerg, sas, brucem

Subscribers: brucem, sas, lldb-commits

Differential Revision: http://reviews.llvm.org/D12750

Change by Kamil Rytarowski <n54@gmx.com>

llvm-svn: 248007