[Debian-science-sagemath] Build failures on arm64 and ppc64el

[Tobias Hansen]

The first results of sage builds on buildds are in.

It failed on arm64 and ppc64el, and I wrote down my findings in this
flint bug report: https://bugs.debian.org/848899

Note that the only release architectures we can target are
amd64, arm64, armhf, i386, mips64el and ppc64el
since on the others openblas is disabled.

Of course getting all these architectures to work is not critical at the
moment (with regard to the freeze).

Best,
Tobias