Documentation for these runners here https://gitlab.arm.com/tooling/gha-runner-docs

Only SLEEF upstream repo is allowed to use these runners.

Integrate runners via strategy.matrix.os, in order to facilitate customisation
of step (e.g. config) based on runner label.

New runners do not seem to come with openssl pre-installed.

Also fix artifacts name for build native, and fix dependencies.

Increase runner size and switch to SVE.

Install OpenSSL since tester3 is enforced.

New runners do not seem to come with openssl or cmake pre-installed.

Also,
- fix comparison for uarch specific configurations
- fix artifacts name for build native.
- use SVE enabled instances (c7g)
- use larger (x->2x) instances so tests run slightly faster without
  compromising on availability.

And update documentation/README.