>Is it just "some upcoming chips" inventing their own extensions? Or is this a standardized ARM extension?

RISC-V has an official ratified extension for TSO, and a work-in-progress one for dynamic switching between RISC-V's standard memory model and TSO.

I'm sorry, I meant RISC-V, not ARM. So far the RISC-V standard has specified behaviour under the TSO memory model and a flag in the ELF header for code that has been compiled for TSO. There is not yet any ratified extension for dynamic switching of memory model but I'd expect anything vendor-specific to be wrapped behind a Linux syscall.