hydra: Really disable 32-to-64-bit cross-builds.

Fixes <http://bugs.gnu.org/26022>.
Reported by Efraim Flashner <efraim@flashner.co.il>.

* build-aux/hydra/gnu-system.scm (hydra-jobs)[cross-jobs](from-32-to-64?):
Check whether TARGET contains "64" rather than checking whether TARGET
ends in "64", which is rarely the case.
(same?): Add special case for armhf-linux.