Add mmtk_collection_in_progress C API

[topolarity]

This API seems sufficient for Julia to implement a "disable-GC-and-wait-for-finish" API, e.g.:

JL_DLLEXPORT void jl_gc_wait_until_disabled(jl_ptls_t ptls)
{
    while (mmtk_collection_in_progress()) {
        mmtk_gc_poll(ptls);
        jl_cpu_pause();
    }
}

Effectively spinlocking on this condition is not great, but I am OK with cleaning that up in a future re-factor.

I'd also be fine to implement #301 or another re-factor instead.

The only trouble is that those APIs seem to focus on disabling GC triggers or disabling GC stop-the-world, which is not sufficient for true GC-must-not-run critical sections. This API is intended to be used if you want to disable new GC and are willing to wait for ongoing GC to finish to make sure it is not running at all.