bifrost fails to build from source on all 32bit architectures as it
requires a 128bit integer type. I think this is ok, but these failures
are slightly unobvious and investigating them can take time to QA
people.

I appreciate if we could skip these builds (also reducing buildd
resource consumption). How about adding architecture-is-64-bit to
Build-Depends? Especially when including a comment, this would make it
very obvious. I'm attaching a patch for your convenience.

Helmut