You can find the full list of known official toolchains here (IPFS).
Upon installation, this toolchain will get installed to /opt/codex by default. For cross compilation, running one of the following commands will set your env up for cross compilation with it:
source /opt/codex/rm10x/3.1.15/environment-setup-cortexa9hf-neon-remarkable-linux-gnueabi source /opt/codex/rm11x/3.1.15/environment-setup-cortexa7hf-neon-remarkable-linux-gnueabi
Please refer to the QT Creator wiki page for more QT specific instructions.
An updated toolchain for Codex version 2.11.0.442 was provided by reMarkable support to a member of the community Discord upon request.
A new toolchain for reMarkable 1 and 2 was announced on the community Discord along with version 2.6. This toolchain continues to work for newer versions of the OS.
The previous 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). The SHA-256 sum of the toolchain is 6299955721bcd9bef92a87ad3cfe4d31df8e2da95b0c4b2cdded4431aa6748b0.
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 It should still work for basic applications, but many libraries are out-of-date which may prevent a binary to run on the reMarkable.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.
Since there is no exact source of the toolchain available, the toltec toolchain was created for the main use of packaging toltec packges. It builds various docker images.
Another community made toolchain can be found here.