Add NetBSD specific version of sanitizer_platform_limits_posix

Summary:
NetBSD is an Open-Source POSIX-like BSD Operating System.

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, fjricci

Reviewed By: kcc

Subscribers: llvm-commits, kubamracek, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D37193

llvm-svn: 311933