Add NetBSD support in asan_stack.h

Summary:
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: vitalybuka

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36377

llvm-svn: 310322