[m-rev.] Upgrade Boehm GC to 7.6.0

[Julien Fischer]

Hi Paul,

On Tue, 20 Sep 2016, Paul Bone wrote:

> For review by anyone
>
> I've tested this with hlc.gc, hlc.par.gc, asm_fast.gc, asm_fast.par.gc.stseg
> on x86_64 Linux.  If anyone would like to test on other platforms that'd be
> good, but it's also okay to test this once its on master.

Go and ahead and commit it to master.  I'll take a look at it on OS X
and Windows after you do that.

Julien.