Detect thread termination in LSan/NetBSD

Summary:
Stop using the Linux solution with pthread_key_create(3).
This approach does not work on NetBSD, because calling
the thread destructor is not the latest operation on a POSIX
thread entity.

Detect _lwp_exit(2) call as it is really the latest operation
called from a detaching POSIX thread.

The pthread_key_create(3) solution also cannot be used
in early libc/libpthread initialization on NetBSD as the
system libraries are not bootstrapped enough.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, kcc, dvyukov

Reviewed By: dvyukov

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40457

llvm-svn: 318994