[Gc] Exporting GC_set_mark_bit and GC_clear_mark_bit

[Petter Urkedal]

On 2012-01-08, Ivan Maidanski wrote:
> I suggest no new typedefs and true/false.
> Please replace:
> GC_bool -> int
> ptr_t -> void*

Done.

> >   - I'll add a comment that the client must hold the allocation lock.
> 
> Of course. (but no need to hurry, the comments may go in a separate commit after we agree about the API changes).

I made another sketch anyway:
https://github.com/paurkedal/bdwgc/commit/9d07ead2d0bc217a3ded434372952f7a8db6c17a

Cheers,
Petter