Add NetBSD LSan support
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
Differential Revision: https://reviews.llvm.org/D64057