Add support for LLVM profile for NetBSD

Summary:
NetBSD uses typical UNIX interfaces.

All tests pass except instrprof-dlopen-dlclose-gcov.test, as there
is not supported semantics of atexit(3) in dlopen(3)ed+dlclose(3)d
DSO.

NetBSD also ships an older version of LLVM profile (ABI v.2 predating
ABI v.4 in upstream version) inside libc. That copy has been manually
removed during the porting and testing process of the upstream version
to NetBSD. Otherwise there were conflicts between them two.

Reviewers: joerg, vitalybuka, vsk

Subscribers: srhines, fedor.sergeev, llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D55997

llvm-svn: 349994