Enable libfuzzer on NetBSD/amd64

Summary:
Enable SanitizerKind::Fuzzer and SanitizerKind::FuzzerNoLink on x86_64.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, vitalybuka, george.karpenkov

Reviewed By: vitalybuka

Subscribers: cfe-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36935

llvm-svn: 311365