atomic operation extensions breaks the cross compilation

Problem has been reported via the mailing list: https://mailman.common-lisp.net/pipermail/ecl-devel/2018-December/011379.html .

I'll take a look myself since I have a CC environment on the second laptop handy.

Generally when we try to cross-compile ECL dpp complains about atomic operations

./configure --host=arm-linux-gnueabi && make

Unknown symbol: mp::compare-and-swap-car 
Unknown symbol: mp::atomic-incf-car
Unknown symbol: mp::compare-and-swap-cdr

This is probably due to not declaring them in one of many places (nb: we should remove this error-prone redundancy from our code at some point of time).