linux: set CONFIG_X86_GENERIC

> This is really intended for distributors who need more generic
> optimizations.

Apparently it's expected that as a generic distribution, we set this,
and if we had we wouldn't have been affected by the recent 32-bit x86
Xen breakage[1][2]:

> In normal distro kernels, CONFIG_X86_GENERIC is enabled

[1]: https://lore.kernel.org/lkml/20231204084722.3789473-1-arnd@kernel.org/
[2]: 096639c548 ("linux: drop XEN on 32-bit")

pkgs/os-specific/linux/kernel/common-config.nix

@@ -123,6 +123,7 @@ let
     };
 
     optimization = {
+      X86_GENERIC = mkIf (stdenv.hostPlatform.system == "i686-linux") yes;
       # Optimize with -O2, not -Os
       CC_OPTIMIZE_FOR_SIZE = no;
     };