Add NetBSD support in sanitizer_procmaps_freebsd.cc

Summary:
This adds NetBSD specific:
 - ReadProcMaps()
 - MemoryMappingLayout::Next()

This code is largely shared with FreeBSD.

Part of the code inspired by the original work on libsanitizer in GCC 5.4 by Christos Zoulas.

Sponsored by <The NetBSD Foundation>

Reviewers: kcc, joerg, filcab, vitalybuka, fjricci

Reviewed By: fjricci

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35551

llvm-svn: 310116