splice.nix: make pkgs splicedPackages when required

This will make `pkgs` used in `callPackage`, and `pkgsCross.X.pkgs` have
packages with `__spliced`.

https://www.github.com/NixOS/nixpkgs/blob/3029741718f4c765fbc5ebf76bea3d6c8ff15fe5/pkgs/development/interpreters/python/passthrufun.nix#L37

https://www.github.com/NixOS/nixpkgs/blob/d2bd9a39dec88eddd5c192abee69939e67f43d12/pkgs/top-level/python-packages.nix#L10720

```
nix-repl>
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
error:
       … while evaluating the attribute
'aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced'
         at
/home/artturin/nixgits/my-nixpkgs/.worktree/1/pkgs/development/python-modules/protobuf/4.nix:119:13:
          118|   passthru = {
          119|     inherit protobuf;
             |             ^
          120|   };

       error: attribute '__spliced' missing
       at «string»:1:1:
            1|
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
             | ^
```

to

```
nix-repl>
pkgsCross.aarch64-multiplatform.python3Packages.protobuf4.protobuf.__spliced
{
  buildBuild = «derivation
/nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildHost = «derivation
/nix/store/s7da5mfvx4h1n86j78knaj9cprglxqz6-protobuf-25.4.drv»;
  buildTarget = «repeated»;
  hostHost = «derivation
/nix/store/mszvybzs4zxh43awyrjnybsfcb265n9r-protobuf-aarch64-unknown-linux-gnu-25.4.drv»;
  hostTarget = «repeated»;
}
```

Co-authored-by: Wolfgang Walther <walther@technowledgy.de>

Author: Artturin

pkgs/development/compilers/gcc/all.nix

@@ -2,6 +2,8 @@
   lib,
   stdenv,
   pkgs,
+  overrideCC,
+  buildPackages,
   targetPackages,
   callPackage,
   isl_0_20,
@@ -62,7 +64,7 @@ let
                 )
                 && stdenv.cc.isGNU
               then
-                pkgs."gcc${majorVersion}Stdenv"
+                overrideCC stdenv buildPackages."gcc${majorVersion}"
               else
                 stdenv;
           }

pkgs/os-specific/windows/default.nix

@@ -27,11 +27,11 @@ lib.makeScope newScope (
       else
         buildPackages.gccWithoutTargetLibc.override (old: {
           bintools = old.bintools.override {
-            libc = pkgs.libc;
+            libc = pkgs.pkgsHostTarget.libc;
             noLibc = pkgs.libc == null;
             nativeLibc = false;
           };
-          libc = pkgs.libc;
+          libc = pkgs.pkgsHostTarget.libc;
           noLibc = pkgs.libc == null;
           nativeLibc = false;
         })

pkgs/top-level/all-packages.nix

@@ -6320,7 +6320,7 @@ with pkgs;
       null;
   bintoolsNoLibc = wrapBintoolsWith {
     bintools = bintools-unwrapped;
-    libc = targetPackages.preLibcHeaders;
+    libc = targetPackages.preLibcHeaders or preLibcHeaders;
   };
   bintools = wrapBintoolsWith {
     bintools = bintools-unwrapped;

pkgs/top-level/splice.nix

@@ -173,6 +173,8 @@ in
 
   newScope = extra: lib.callPackageWith (pkgsForCall // extra);
 
+  pkgs = if actuallySplice then splicedPackages // { recurseForDerivations = false; } else pkgs;
+
   # prefill 2 fields of the function for convenience
   makeScopeWithSplicing = lib.makeScopeWithSplicing splicePackages pkgs.newScope;
   makeScopeWithSplicing' = lib.makeScopeWithSplicing' {