[v2] Windows native GDB event handling enhancement

[Joel Brobecker <brobecker@adacore.com>]

Hello,

This is v2 of the patch series, which now consists of 2 patches:

 * [RFA v2 1/2] (Windows) fix thr != nullptr assert failure in

   Compared to v1, it has been reduced to the essence of the fix,
   which is to avoid using main_thread_id during the exit-process
   event; and that's it.

 * [RFA v2 2/2] gdb/windows-nat.c: Get rid of main_thread_id global

   That one shold be unchanged.

The part that re-instated the thread notifications for the main thread
(at create-process event) and for the last thread (at exit-process
event) has been excised from v1 of patch #1.  I think this is the best
compromise based on the discussions so far.

I will personally miss the silenced notifications, but for those
like me, we can enable the windows event debug logs.

The reason why I kept the patches split in two is so that I can apply
patch #1 on the gdb-8.3 branch as well. Patch #2 has been behaving
well as far as AdaCore nightly testing is concerned, but it's only
been a few weeks since we implemented it, and I think it deserves
more time in master before being in a release.

The patches were tested on both 32bit and 64bit native windows,
using AdaCore testsuite. OK to push?

Thanks,
-- 
Joel