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

pkgs/development/compilers/gcc/7/default.nix

@@ -289,7 +289,7 @@ stdenv.mkDerivation ({
     optional (!bootstrap) "--disable-bootstrap" ++
 
     # Platform-specific flags
-    optional (targetPlatform == hostPlatform && targetPlatform.isi686) "--with-arch=i686" ++
+    optional (targetPlatform == hostPlatform && targetPlatform.isx86_32) "--with-arch=${stdenv.hostPlatform.parsed.cpu.name}" ++
     optionals hostPlatform.isSunOS [
       "--enable-long-long" "--enable-libssp" "--enable-threads=posix" "--disable-nls" "--enable-__cxa_atexit"
       # On Illumos/Solaris GNU as is preferred