Add NetBSD support in sanitizer_procmaps_common.cc

Summary:
Reuse Linux and FreeBSD code - 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, kcc, vitalybuka, filcab

Reviewed By: filcab

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35632

llvm-svn: 308614