Add NetBSD support in asan_linux.cc

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: fjricci, vitalybuka, joerg, kcc, filcab

Reviewed By: vitalybuka

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36488

llvm-svn: 310647