The `arch`/`opsys` testsuite predicates are inconsistent and confusing

When testing cross GHC, arch actually represents target arch, which is desirable, but opsys represents host OS! This is inconsistent and confusing, and a lot of existing when(opsys(...), ...) logic in the testsuite is written without cross compilation in mind.

Should we refactor these predicates to take host/target prefixes?

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