Add NetBSD syscall hooks skeleton in sanitizers

Summary:
Implement the skeleton of NetBSD syscall hooks for use with sanitizers.

Add a script that generates the rules to handle syscalls
on NetBSD: generate_netbsd_syscalls.awk. It has been written
in NetBSD awk(1) (patched nawk) and is compatible with gawk.

Generate lib/sanitizer_common/sanitizer_platform_limits_netbsd.h
that is a public header for applications, and included as:
<sanitizer_common/sanitizer_platform_limits_netbsd.h>.

Generate sanitizer_syscalls_netbsd.inc that defines all the
syscall rules for NetBSD. This file is modeled after the Linux
specific file: sanitizer_common_syscalls.inc.

Start recognizing NetBSD syscalls with existing sanitizers:
ASan, ESan, HWASan, TSan, MSan.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, kcc, dvyukov, eugenis

Reviewed By: vitalybuka

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

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D42048

llvm-svn: 325206