Correct atexit(3) support in TSan/NetBSD

Summary:
The NetBSD specific implementation of cxa_atexit() does not
preserve the 2nd argument if dso is equal to NULL.

Changes:

 - Split paths of handling intercepted __cxa_atexit() and atexit(3).
   This affects all supported Operating Systems.
 - Add a local stack-like structure to hold the __cxa_atexit() context.
   atexit(3) is documented in the C standard as calling callback from the
   earliest to the oldest entry. This path also fixes potential ABI
   problem of passing an argument to a function from the atexit(3)
   callback mechanism.
 - Add new test to ensure LIFO style of atexit(3) callbacks: atexit3.cc

Proposal to change the behavior of __cxa_atexit() in NetBSD has been rejected.

With the above changes TSan/NetBSD with the current tsan_interceptors.cc
can bootstrap into operation.

Sponsored by <The NetBSD Foundation>

Reviewers: vitalybuka, dvyukov, joerg, kcc, eugenis

Reviewed By: dvyukov

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D39619

llvm-svn: 317735