Introduce SANITIZER_NETBSD in sanitizer_platform.h

Summary:
Add defines for new NetBSD: SANITIZER_NETBSD,
it will be used across the codebase for sanitizers.

NetBSD is a POSIX-like platform, add it to SANITIZER_POSIX.

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, dim, alekseyshl, filcab, eugenis, vitalybuka

Reviewed By: kcc

Subscribers: kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35467

llvm-svn: 308216