tests fail with `-march=x86-64-v3` (and s390x)

[TheJJ]

hey!
The Ubuntu package for pybdsf on 26.04 unfortunately has test failures when built with amd64v3 or s390x instruction set (launchpad bug) with gcc 15.2.0-12ubuntu1.

reproduce:

% lxc launch ubuntu-daily:resolute pybdsf-test
% lxc exec pybdsf-test -- sudo -i -u ubuntu bash
$ git clone https://git.launchpad.net/~jj/ubuntu/+source/pybdsf
$ cd pybdsf
$ git switch --detach origin/import/1.13.0-3build2
$ sudo apt install dpkg-dev build-essential debhelper-compat=13 dh-python meson pybuild-plugin-pyproject python3-all-dev python3-astropy python3-setuptools-scm python3-numpy python3-numpy-dev python3-scipy python3-skbuild libboost-python-dev libboost-numpy-dev gfortran
$ DEB_HOST_ARCH_VARIANT=amd64v3 dpkg-buildpackage -b -us -uc

which should fail with:

$ cd .pybuild/cpython3_3.13/build/test
$ PYTHONPATH=../python3.13 tbdsf_process_image.py
[...]
ERROR [root]: Image files 'tbdsf_process_image.in_fits_pybdsf/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_maj.fits' and 'reference_outputs/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_maj.fits' differ (rtol=0.01)
Noise: 1843.770 mJy/b vs 2016.463 mJy/b
ERROR [root]: Image files 'tbdsf_process_image.in_fits_pybdsf/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_min.fits' and 'reference_outputs/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_min.fits' differ (rtol=0.01)
Noise: 2267.707 mJy/b vs 1778.006 mJy/b
ERROR [root]: Image files 'tbdsf_process_image.in_fits_pybdsf/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_pa.fits' and 'reference_outputs/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_pa.fits' differ (rtol=0.01)
Noise: 12088.033 mJy/b vs 8427.595 mJy/b
ERROR [root]: Image files 'tbdsf_process_image.in_fits_pybdsf/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_ratio.fits' and 'reference_outputs/misc/tbdsf_process_image.in_fits.pybdsf.psf_vary_ratio.fits' differ (rtol=0.01)
Noise: 26.648 mJy/b vs 81.115 mJy/b
ERROR [root]: Image files 'tbdsf_process_image.in_fits_pybdsf/misc/tbdsf_process_image.in_fits.pybdsf.volrank.fits' and 'reference_outputs/misc/tbdsf_process_image.in_fits.pybdsf.volrank.fits' differ (rtol=0.01)
Noise: 1461.902 mJy/b vs 1712.265 mJy/b

reference-tbdsf_process_image.in_fits.pybdsf.volrank.fits:

testresult-tbdsf_process_image.in_fits.pybdsf.volrank.fits:

if you build it with dpkg-buildpackage -b -us -uc (i.e. -march=x86-64 without v3) there's no error.

the same seems to happen on s390x as well.