multiprocessing: implement atomic operations

Implement atomic operations:

atomic-incf, atomic-decf and compare-and-swap (or conditional-store as it's named in the CLX). We already depend on boehm-gc, which pulls libatomic_ops, so this shouldn't be a big problem.

(mp:atomic-incf place 3)
(mp:atomic-decf place 3)
(mp:compare-and-swap place old-value new-value)

It would be nice to have it in both multiprocessing and non-thread builds (on the latter these would be dull macros for incf, decf and

(when (eq ,place ,old-value)
       (setf ,place ,new-value)
       t)

if so, maybe it would be good to put them in ext? I think not, because without-interrupts is already in the mp package.

Added documentation label

changed milestone to %ECL 16.2.0