Some 32-bit platforms need explicit linking to `libatomic` for 8-byte atomics support

Some 32-bit platforms (`ppc`, `arm`, likely `mips`) need explicit linking to `libatomic` here:

```

[100%] Linking CXX executable FuzzFailTest

cd /opt/local/var/macports/build/_opt_PPCSnowLeopardPorts_devel_CLI11/CLI11/work/build/tests && /opt/local/bin/cmake -E cmake_link_script CMakeFiles/FuzzFailTest.dir/link.txt --verbose=ON

/opt/local/bin/g++-mp-13 -pipe -Os -DNDEBUG -I/opt/local/include -D_GLIBCXX_USE_CXX11_ABI=0 -mmacosx-version-min=10.6 -Wl,-search_paths_first -Wl,-headerpad_max_install_names -L/opt/local/lib -Wl,-headerpad_max_install_names CMakeFiles/FuzzFailTest.dir/FuzzFailTest.cpp.o CMakeFiles/FuzzFailTest.dir/__/fuzz/fuzzApp.cpp.o -o FuzzFailTest  -Wl,-rpath,/opt/local/lib /opt/local/lib/libCatch2Main.a /opt/local/lib/libCatch2.a 

Undefined symbols:

  "___atomic_store_8", referenced from:

      __ZNSt17_Function_handlerIFbRKSt6vectorISsSaISsEEEZN3CLI3App8add_flagISt6atomicIdELNS6_6detail7enablerE0EEEPNS6_6OptionESsRT_SsEUlS4_E_E9_M_invokeERKSt9_Any_dataS4_ in fuzzApp.cpp.o

      __ZNSt17_Function_handlerIFbRKSt6vectorISsSaISsEEEZN3CLI3App10add_optionISt6atomicIxESA_LNS6_6detail7enablerE0EEEPNS6_6OptionESsRT_SsEUlS4_E_E9_M_invokeERKSt9_Any_dataS4_ in fuzzApp.cpp.o

      __ZNSt17_Function_handlerIFbRKSt6vectorISsSaISsEEEZN3CLI3App10add_optionISt6atomicIyESA_LNS6_6detail7enablerE0EEEPNS6_6OptionESsRT_SsEUlS4_E_E9_M_invokeERKSt9_Any_dataS4_ in fuzzApp.cpp.o

  "___atomic_load_8", referenced from:

      __ZNSt17_Function_handlerIFSsvEZN3CLI3App10add_optionISt6atomicIxES5_LNS1_6detail7enablerE0EEEPNS1_6OptionESsRT_SsEUlvE_E9_M_invokeERKSt9_Any_data in fuzzApp.cpp.o

      __ZNSt17_Function_handlerIFSsvEZN3CLI3App10add_optionISt6atomicIyES5_LNS1_6detail7enablerE0EEEPNS1_6OptionESsRT_SsEUlvE_E9_M_invokeERKSt9_Any_data in fuzzApp.cpp.o

ld: symbol(s) not found

collect2: error: ld returned 1 exit status

make[2]: *** [tests/FuzzFailTest] Error 1

```

[]