Update RISC-V's support tier in `configure.ac`

# Feature or enhancement

### Proposal:

It's now a Tier 3 platform: https://github.com/python/steering-council/issues/360


### Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

### Links to previous discussion of this feature:

_No response_


### Linked PRs
* gh-156277