Make generic atomicity.h use gthr.h mutexes

[Loren James Rittle]

>2002-11-07  Phil Edwards  <pme@gcc.gnu.org>
>            Richard Earnshaw  <rearnsha@arm.com>
>
>	* atomicity.h:  Provide atomic __exchange_and_add and __atomic_add.

This patch has a slight problem in that you can't assume that a gthr.h
mutex may be statically init'd.  You need to handle systems (esp. in
the so-called generic code) that only support dynamic init.  Once you
see how complex this is to get perfectly correct, you will understand
why I have not volunteered to produce this patch... ;-)