arch.7: update 11.x to 11.4 as the last FreeBSD 11 release

armeb and pc98 were both discontinued after FreeBSD 11. FreeBSD 11.4
is now known to be the final 11.x release, so update to the specific
version.

(cherry picked from commit 4f9548640dd1c7e9602f0d01647b112e51792224)