link with libatomic on architectures that require it

link with libatomic on architectures that require it

Last-Update: 2020-03-11
Forwarded: no

clingo requires c++11 threads with 64bit __atomic_exchange, we need to
link with libatomic on armel, powerpc, powerpcspe, m68k, mips, mipsel,
and sh4, see also https://gcc.gnu.org/wiki/Atomic and
https://gcc.gnu.org/wiki/Atomic/GCCMM/LIbrary

This patch was shamelessly adapted from
https://github.com/potassco/clasp/blob/master/CMakeLists.txt
https://github.com/potassco/clasp/commit/fe3e176e2813956000bb947479de2c3d80fe34be

Gbp-Pq: Name link-libatomic-check.patch