Add NetBSD support in platform_interceptors.h

Summary:
Introduce SI_NETBSD for NetBSD.

Add NetBSD support for appropriate `SANITIZER_INTERCEPT_*`.

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, dim, kcc, alekseyshl, filcab, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35468

llvm-svn: 308217