Correct atexit(3) support in MSan/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.
 - Allow usage of global vars with ctors in interceptors.
   This allows to use Vector without automatic cleaning up the structures.

This code has been modeled after TSan implementation for the same functions.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, dvyukov, eugenis, vitalybuka, kcc

Reviewed By: vitalybuka

Subscribers: delcypher, devnexen, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40714

llvm-svn: 346579