BUG: fix support for gufuncs with more than one core dimension

[mhvk]

In particular, erfa.rxp and erfa.rxr.

This just fixes the bug, and is slightly weird in its treatment of axis. I have a follow-up PR in the works which just adds proper support for axes - but that would be less suitable for backporting.

Fixes #16116

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[neutrinoceros]

very nice !

[neutrinoceros]

I find it pretty elegant for a hack :-)
More importantly, it does fix the issue in a back portable way, so that's more than enough for me. Thank you very much for going at it so quickly !

[neutrinoceros]

Actually, now I'm seeing a new error in MaskedArray.__array_ufunc__ in #16070 (which this PR is supposed to unblock) https://github.com/astropy/astropy/actions/runs/8076412008/job/22064849015?pr=16070

I don't know yet if that's a new bug in this PR or if it's also present on main (will inspect). I just wanted to hold off this one until we know more. I'll update my review status as soon as possible !

[neutrinoceros]

Okay, so what I'm seeing on #16070 is indeed another, unrelated (but adjacent) bug, see #16123

I don't know that it should be fixed alongside #16116 (maybe so ?), in any case, I think it's okay to merge this one as is. Thanks again !

[mhvk]

@pllim - this was reviewed by @neutrinoceros, which I think should suffice. Could you merge for us?

[pllim]

Approving by proxy. Thanks!

(I don't grok this but I trust you...)