A slight correction (I made it earlier but didn't highlight it): 5a is also used for Thumb code, as "5a -t" to restrict the input to thumb constructions and tag the output as Thumb code (using .t files instead of .5)
The thumb mkfile sets AS=5a -t
Since the Thumb-2 changes aren't supported, and it's the original Thumb, I think that makes the supported architecture Armv6-M in current documentation terms.
It looks to me as though it would work on Cortex M0, but I think the thumb mode was only ever used by us on ARM's little Armv7-t Evaluator board, which I can't even find documented now.
Normally even that board ran in (normal) ARM32 mode. The Thumb work on it was just a stepping stone for using Thumb mode on a proprietary 3rd party ARM implementation, although even there it was probably experimental.
Inferno can run without an MMU. Even given an MMU it sets a fairly flat map across physical memory and the MMU functionality is mainly used to map devices.
The os/ks32 directory is a guide to squashing the system down to fit modest if not tiny memory (again, in ARM32 mode).