Add NetBSD LSan support

Summary:
Combine few relatively small changes into one:

 - implement internal_ptrace() and internal_clone() for NetBSD
 - add support for stoptheworld based on the ptrace(2) API
 - define COMPILER_RT_HAS_LSAN for NetBSD
 - enable tests for NetBSD/amd64

Inspired by the original implementation by Christos Zoulas in netbsd/src for GCC.

The implementation is in theory CPU independent through well defined macros
across all NetBSD ports, however only the x86_64 version was tested.

Reviewers: mgorny, dvyukov, vitalybuka, joerg, jfb

Reviewed By: vitalybuka

Subscribers: dexonsmith, jfb, srhines, kubamracek, llvm-commits, christos

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64057

llvm-svn: 365735