32-bit platforms need linking to `libatomic` to support 8-byte atomic builtins

[barracuda156]

google-benchmark needs linking to libatomic on 32-bit platforms:

/opt/local/bin/g++-mp-13 -pipe -Os -DNDEBUG -I/opt/local/include -D_GLIBCXX_USE_CXX11_ABI=0 -std=c++11 -fno-operator-names -arch ppc -mmacosx-version-min=10.6 -Wl,-search_paths_first -Wl,-headerpad_max_install_names -L/opt/local/lib -Wl,-headerpad_max_install_names CMakeFiles/aero_benchmarks.dir/benchmark_aero.cc.o -o aero_benchmarks   -L/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_iv/iv/work/iv-64c3a9c7c517063f29d90d449180ea8f6f4d946f/third_party/google-test  -L/opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_iv/iv/work/iv-64c3a9c7c517063f29d90d449180ea8f6f4d946f/third_party/google-benchmark  -Wl,-rpath,/opt/local/lib ../../../third_party/google-benchmark/libgoogle-benchmark.a //usr/lib/libm.dylib 
Undefined symbols:
  "___atomic_load_8", referenced from:
      __ZN9benchmark5State11KeepRunningEv in libgoogle-benchmark.a(benchmark.cc.o)
  "___atomic_store_8", referenced from:
      __ZN9benchmark5State12StartRunningEv in libgoogle-benchmark.a(benchmark.cc.o)
      __ZN9benchmark5State9FastClock8BGThreadEv in libgoogle-benchmark.a(benchmark.cc.o)
ld: symbol(s) not found

P. S. I understand it is a third-party library, but since it is bundled, it makes sense to fix it locally.