[m-rev.] upgrade Boehm GC to v7.6.6 + patch

[Peter Wang]

On Mon, 7 May 2018 12:45:36 +1000, Peter Wang <novalazy at gmail.com> wrote:
> Hi,
> 
> I plan to switch the boehm_gc submodule to release-7_6-mercury-20180223.
> It includes an important fix for Windows 10 (April 2018 update);
> without the fix, any Mercury programs will crash on that OS.
> 
> That branch is also updated to Boehm GC to v7.6.6 from v7.6.4.
> 
> As there are a number of changes pending today, I'll hold off switching
> the boehm_gc submodule until tommorrow or the day after.

Update your workspaces to the correct Boehm GC version by running:

 git submodule update --recursive

Then "git submodule status --recursive" should report:

 b08a6f1ff8f7c54bcea46dc32d85ae770a7d3ebb boehm_gc (release-7_6-mercury-20180507)
 b1fe74532b24e25e1ad944df4af11d958b0ad53d boehm_gc/libatomic_ops (release-7_6-mercury-20180108)

Peter