FreeBSD calls "x86_64" "amd64"

[geekosaur]

This prepares for a "tier 2 platforms" validate job: currently there are platforms for which we make official releases, but never validate beyond building them.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

Depends-on: #10405 (note: includes its commit, which should drop out when this lands)

[geekosaur]

Made irrelevant by #10320, which uses ghc's canonicalization of the arch.