Don't crash when target platform is not supported

[arjan]

This addresses both #89 and #90.

On compile time, a warning is printed when the CPU platform is not supported.

On runtime, while loading the NIF, an error is printed. All NIF
stub functions now return stub values and do not call exit() so that
the integration keeps working (without doing anything) when running on
a non-supported OS.

@jeffkreeftmeijer can you test this somehow?

[arjan]

You can partially test this by returning :unsupported in mix_helpers.exs for all archs.
However, the JSON encoding tests will fail. I'm wondering whether we want to expose this data_to_json function in the Appsignal API at all..? People might misuse it as a generic JSON encoding mechanism.

[jeffkreeftmeijer]

Tested this, as you suggested, by returning :unsupported from Mix.Appsignal.Helpers.map_arch/1.

The data_to_json() function is specifically added to the agent to help with testing the C extension and Nif, and won’t be used elsewhere. You’re right, we shouldn’t export it in the Nif, at least.