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

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@312183 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/FuzzerExtraCounters.cpp b/FuzzerExtraCounters.cpp
index 07dbe0f..0e7a776 100644
--- a/FuzzerExtraCounters.cpp
+++ b/FuzzerExtraCounters.cpp
@@ -11,7 +11,7 @@
 
 #include "FuzzerDefs.h"
 
-#if LIBFUZZER_LINUX
+#if LIBFUZZER_LINUX || LIBFUZZER_NETBSD
 __attribute__((weak)) extern uint8_t __start___libfuzzer_extra_counters;
 __attribute__((weak)) extern uint8_t __stop___libfuzzer_extra_counters;