[boehms-gc, libjava?] Update Boehm's GC to 6.8

[Laurynas Biveinis]

> The approximately equivalent code in the atomic_ops package, and hence
> in 7.0, reads:

[...]

So this is the same as in already commited 6.8 sources, no need to
change anything, I think.

-- 
Laurynas