linuxManualConfig: set badPlatforms

Older kernels shouldn't be marked as supporting architectures that
were only added to the kernel later.

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

···
       maintainers.thoughtpolice
     ];
     platforms = platforms.linux;
+    badPlatforms =
+      lib.optionals (lib.versionOlder version "4.15") [ "riscv32-linux" "riscv64-linux" ] ++
+      lib.optional (lib.versionOlder version "5.19") "loongarch64-linux";
     timeout = 14400; # 4 hours
   } // extraMeta;
 } // optionalAttrs (pos != null) {