Add UEF as a target

[sparques]

[aykevl]

For this PR, I have the same reservations as for #3996:

• It's very big, and therefore hard to review. I'd prefer to start with something that is minimal, to which we can add more over time.
• I'd prefer if this was written like a baremetal target, and not something separate. Baremetal targets (that set the baremetal build tag etc) are easier to maintain since they can be treated similar to a microcontroller from a runtime/stdlib perspective.