linux: add feature flag to indicate support for 32bit emulation

Motivated by the need to warn users trying to build configurations that depend
on being able to run 32bit apps on 64bit kernels.

pkgs/os-specific/linux/kernel/generic.nix

@@ -62,6 +62,7 @@ let
     netfilterRPFilter = true;
     grsecurity = false;
     xen_dom0 = false;
+    ia32Emulation = true;
   } // features) kernelPatches;
 
   intermediateNixConfig = import ./common-config.nix {

pkgs/top-level/all-packages.nix

@@ -14825,6 +14825,7 @@ in
 
   # Hardened linux
   hardenedLinuxPackagesFor = kernel: linuxPackagesFor (kernel.override {
+    features.ia32Emulation = false;
     extraConfig = import ../os-specific/linux/kernel/hardened-config.nix {
       inherit stdenv;
       inherit (kernel) version;