Can we use Rust / Go / Java (GraalVM Native) or Flutter Linux to build an executable that runs on an OS with an Ironclad kernel? Or is there special treatment that makes it incompatible with "plain" Linux exe?
Ironclad has a POSIX API, so yes, in principle it should be possible. There is an OS on top of Ironclad (https://github.com/Ironclad-Project/Gloire) which uses GNU tools.