What is the best way to restrict building a package to 64-bit platforms?

I have a package (ruby-commonmarker) that does not work correctly on 32-bit 
platforms (builds but fails during test, not supported upstream on 32-bit).  
What is the best way to prohibit building on 32-bit systems?

I know I could make a manual list of all of the currently-supported 64-bit 
platforms and put those in the Architecture field, but that seems fragile to 
me, as it would require manual updates every time a new 64-bit platform was 
added.

I looked at Architecture wildcards, but I couldn’t see a way to use them to 
specify all 64-bit platforms.

https://www.debian.org/doc/debian-policy/ch-customized-programs.html#s-arch-wildcard-spec

I have a vague recollection that there is a pseudo-package that only exists on 
64-bit platforms, and you can restrict a package to only attempt 64-bit builds 
by Build-Depending on that package, but I don’t remember what the name was, 
and I haven’t been able to discover it by searching.

Can anyone point me in the right direction?

-- 
Soren Stoutner
[email protected]

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