Add NetBSD support in sanitizer_errno.h

Summary:
NetBSD ships with __errno (value for __errno_location) like Android.

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, vitalybuka, fjricci, kcc, filcab

Reviewed By: vitalybuka

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36360

llvm-svn: 310182