The latest official toolchain is oecore-x86_64-cortexa9hf-neon-toolchain-zero-gravitas-1.8-23.9.2019.sh it was originally hosted on the reMarkable engineering site, but this URL went down. You can download it either from the Internet Archive or IPFS (hash: QmZmt4UtvyLLA8mLde6WspqvhMAKjzfvJW91R3bEja6y3A) (Alternate Link).
Upon installation, this toolchain will get installed to /usr/local/oecore-x86_64 by default. For cross compilation, running the following command will set your env up for cross compilation with it:
Please refer to the QT Creator wiki page for more QT specific instructions.
Another community made toolchain can be found here.
The first official toolchain was called poky. It should still work for basic applications, but many libraries are out-of-date which may prevent a binary to run on the reMarkable. It can be found over the Internet Archive or IPFS (hash: Qmdkdeh3bodwDLM9YvPrMoAi6dFYDDCodAnHvjG5voZxiC) (Alternate Link).
Some projects will still refer to this toolchain and should get updated to use the official oecore toolchain instead.