Add NetBSD support in sanitizer_linux_libcdep.cc

Summary:
When possible reuse FreeBSD and Linux code.

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, vitalybuka, filcab

Reviewed By: vitalybuka

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36320

llvm-svn: 310143