There aren't any ARM7 specific libraries for ThingSpeak that I know of. But I wouldn't let that stop you. All you need an HTTP client library, of which there are many. Then just use the HTTP library to send GET or POST requests to ThingSpeak with your API key and data.
Alternately, you can also interact with ThingSpeak using MQTT.
ThingSpeak is specifically designed for low performance hardware, so the reading or writing data is very straightforward.
Take a look at the documentation to see how to construct the requests.