Add riscv64 to the library

[athomas]

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[cpswan]

@athomas I can see Trixie based builds are now flowing through for 3.9.0 etc., but so far no sign of riscv64 images :( Is there another bit that needs to be flipped somewhere?

[athomas]

@cpswan I don't think so, I think the official build is simply not finished, yet: https://doi-janky.infosiftr.net/job/meta/job/riscv64/job/build/13347/

Looks like quite a few builds are queued up. So perhaps they have limited capacity for riscv64. Let's give it another day or two.