Argonaut RISC Core/arc architecture unsupported

[jameshilliard]

I'm seeing this error when building greenlet using buildroot's ci here.

greenlet needs to be ported to this platform, or teached how to detect your compiler properly.

If supporting this architecture is not possible I can flag greenlet as incompatible but it would be ideal to add support.

[jamadden]

PRs are welcome!