This commit tries to make it simpler to add unit tests (cctest) for
code that needs to test node core funtionality but that might not be
appropriate as an addon or a JavaScript test. An example of this could
be adding functionality targeted for situations when Node itself is
embedded.

Currently it was not as easy, or efficient, as one would have hoped to
add such tests. The object output directories vary for different
operating systems which we need to link to so that we don't have an
additional compilation step.

PR-URL: nodejs#11956
Ref: nodejs#9163
Reviewed-By: James M Snell <jasnell@gmail.com>

This commit attempts to address one of the TODOs in
nodejs#4641 regarding making the
AtExit callback's per environment, instead of the current global.

bnoordhuis provided a few options for solving this, and one was to
use a thread-local which is what this commit attempts to do.

PR-URL: nodejs#9163
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>