Plasma and Nitro need to support building on ARM64

Attempting to build Server on AWS Graviton ARM architecture, I'm getting compilation failures like:
{{# github.com/couchbase/plasma
../../../../../../goproj/src/github.com/couchbase/plasma/page.go:158:37: pidToSKNode(pid).GetNext undefined (type *skiplist.Node has no field or method GetNext)
../../../../../../goproj/src/github.com/couchbase/plasma/pagetable.go:135:3: n.SetItem undefined (type *skiplist.Node has no field or method SetItem)
../../../../../../goproj/src/github.com/couchbase/plasma/swapper.go:1731:12: n.Cache undefined (type *skiplist.Node has no field or method Cache)}}

{{# github.com/couchbase/nitro/skiplist
../../../../../../goproj/src/github.com/couchbase/nitro/skiplist/skiplist.go:102:5: undefined: debugMarkFree}}

Sarath Lakshman mentioned this may date back to when 32-bit support was a thing and specific 64-bit functionality was added. He provided a work-around for nitro (http://review.couchbase.org/c/nitro/+/156535 ) which let me get as far as the plasma errors.