clarify status of ARM and 32-bit architectures

The README says ARM is unsupported and 32-bit untested. At least for Apple M series ARM this seems to be out of date. (It looks like Github's CI runners for MacOS are actually M1, so there is evidence in our own actions.)

Do other ARM systems support libquadmath?

Has anyone tried to run the tests on 32-bit systems recently (apparently Github doesn't provide these)?

Start with the README statements about ARM and 32-bit support, then inspect the GitHub Actions configuration for Apple M-series evidence. Check whether libquadmath is available on relevant ARM systems and whether recent 32-bit test results exist. Done means documenting verified support and testing status.

Written by the indexing model from the issue text.