openssl: don't autodetect platform on armv6/7l

[lopsided98]

Motivation for this change

OpenSSL currently generates invalid armv6l instructions when compiled on an armv7l machine because it uses its own system for detecting the architecture, so the autotools build_alias trick does not work. This type of build seems to be a little controversial and unsupported in nixpkgs, but I think this PR is harmless and makes the build more pure.

Things done

This PR explicitly specifies the architecture on armv6l and armv7l, causing the correct instructions to be generated even when building for armv6l on armv7l. This change does not affect other architectures.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.

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