Make generic atomicity.h use gthr.h mutexes

[B. Kosnik]

>If the user feels that correct operation is too slow, then --disable-threads
>will turn the mutex calls into no-ops, and the memory writes will be
>non-atomic, which is exactly what we have now.

... and I continue to think that this proposed behavior is better than
the status quo. 

I think your patch is a good thing, and that it should be supplemented
with a documentation patch that explains what is going on, or perhaps a
warning when configure.target finds out that the generic code path for
atomicity.h is going to be used.

I think as long as you document how to turn off atomic ops, Nathan's
opposition will fade.

-benjamin