Some architectures, specifically armel, mipsel, m68k & powerpc in
Debian, need to explicitly link to atomic.
Use meson to see if the target's CPU family is one of those, and if so,
pass -latomic to the linker.
Tested on armel and mipsel machines to verify passing -latomic works, and
on armhf and amd64 to ensure normal builds aren't broken.
Fixes#371.