Add NetBSD support in sanitizer_unwind_linux_libcdep.cc

Summary:
NetBSD is a POSIX-like and BSD-family system.

Reuse FreeBSD and Linux code.

NetBSD uses DWARF ExceptionHandler.

Part of the code inspired by the original work on libsanitizer in GCC 5.4 by Christos Zoulas.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, filcab, vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, emaste, llvm-commits, kubamracek, aprantl, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36314

llvm-svn: 310179