Add NetBSD support in sanitizer_procmaps.h

Summary:
Reuse Linux and FreeBSD - no NetBSD specific changes.

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, filcab, kcc, vitalybuka

Reviewed By: filcab

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35629

llvm-svn: 308615