Include arch-specific data for NDBM and GDBM autopkgtests

Skip the test if no data exists for the architecture being tested.

The ppc64el, arm64, s390x and arm64 databases were manually created on
Debian porter machines (plummer, amdahl, zelenka and harris.)

We're not providing data for stretch or jessie on 32-bit architectures
because libgdbm dropped compatibility. See #923609.

Closes: #923409