this architecture mapping is used broadly in the node ecosystem.
an assortment of tools and hooks, like buildNpmPackage or
pnpm.configHook, will benefit from reusing these values. placing them in
stdenv makes sense because (1) several of these tools don't currently
depend on nodejs, and may even be available where nodejs is not and (2)
`stdenv.{build,host,target}Platform` seems to be less error-prone than
`pkgs*.nodejs.{os,arch}` -- especially for setup hooks where the offsets
are shifted.
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