Probably just Build-Depends: pocl-opencl-icd [amd64 arm64 armel armhf i386 hurd-i386] and skip the tests on the other architectures. (Linking uses libopencl1, not *-opencl-icd.)
As a slightly unrelated note, if that Build-Depends is only used during testing, than please also mark it with the <!nocheck> build profile to document that fact. That makes reasoning about these kind of things easier for non-experts.
Paul
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