add generic x86_32 support (#52634)

* add generic x86_32 support

- Add support for i386-i586.
- Add `isx86_32` predicate that can replace most uses of `isi686`.
- `isi686` is reinterpreted to mean "exactly i686 arch, and not say i585 or i386".
- This branch was used to build working i586 kernel running on i586 hardware.

* revert `isi[345]86`, remove dead code

- Remove changes to dead code in `doubles.nix` and `for-meta.nix`.
- Remove `isi[345]86` predicates since other cpu families don't have specific model predicates.

* remove i386-linux since linux not supported on that cpu

lib/systems/inspect.nix

@@ -9,7 +9,8 @@ let abis = lib.mapAttrs (_: abi: builtins.removeAttrs abi [ "assertions" ]) abis
 rec {
   patterns = rec {
     isi686         = { cpu = cpuTypes.i686; };
-    isx86_64       = { cpu = cpuTypes.x86_64; };
+    isx86_32       = { cpu = { family = "x86"; bits = 32; }; };
+    isx86_64       = { cpu = { family = "x86"; bits = 64; }; };
     isPowerPC      = { cpu = cpuTypes.powerpc; };
     isPower = { cpu = { family = "power"; }; };
     isx86          = { cpu = { family = "x86"; }; };

lib/systems/platforms.nix

@@ -467,6 +467,8 @@ rec {
   };
 
   selectBySystem = system: {
+      "i486-linux" = pc32;
+      "i586-linux" = pc32;
       "i686-linux" = pc32;
       "x86_64-linux" = pc64;
       "armv5tel-linux" = sheevaplug;