Motivation for this change

On armv6l, grpc needs to link to libatomic to provide C++ atomic operations. Without this PR, importing this Python module results in the following error:

Traceback (most recent call last):
  File "nix_run_setup", line 8, in <module>
    exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\\r\\n', '\\n'), __file__, 'exec'))
  File "setup.py", line 6, in <module>
    from grpc_tools import protoc
  File "/nix/store/b21rflbwjpfj7lxm3fg88p64ri6p0pm6-python3.7-grpcio-tools-1.23.0/lib/python3.7/site-packages/grpc_tools/protoc.py", line 20, in <module>
    from grpc_tools import _protoc_compiler
ImportError: /nix/store/b21rflbwjpfj7lxm3fg88p64ri6p0pm6-python3.7-grpcio-tools-1.23.0/lib/python3.7/site-packages/grpc_tools/_protoc_compiler.cpython-37m-arm-linux-gnueabihf.so: undefined symbol: __atomic_fetch_add_8

One might expect that the grpcio package would need this fix, rather than grpcio-tools, but this does not seem to be the case.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @vanschelven