Force positive stackprotector detection on x64 for Linux >=4.16

[clefru]

The stackprotector is not detected correctly when during out-of-kernel compilations, breaking ZFS under >=4.16.

Motivation for this change

Fixes #39225

Things done

• 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)
  • nix-build -A stable zfs.nix for kernelPackage forced to pkgs.linuxPackages_4_16 and pkgs.linuxPackages_4_17.
• 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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[clefru]

I have a better fix than this.