Add preliminary NetBSD support in libfuzzer

Summary:
This code already works and passes some number of tests.

There is need to finish remaining sanitizers to get better coverage.

Many tests fail due to overly long file names of executables (>31).
This is a current shortcoming of the NetBSD 8(beta) kernel, as
certain functions can fail (like retrieving file name of executable).

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, vitalybuka, george.karpenkov

Reviewed By: kcc

Subscribers: mgorny, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D37304

llvm-svn: 312183