stdenv/common-path.nix: move to stdenv/generic/

Do this since it is part of the generic stdenv/`mkDerivation`
infrastructure, rather than being a bootstrapping strategy.

pkgs/misc/my-env/default.nix

@@ -73,7 +73,7 @@ mkDerivation {
   };
 
   buildPhase = let
-    initialPath = import ../../stdenv/common-path.nix { inherit pkgs; };
+    initialPath = import ../../stdenv/generic/common-path.nix { inherit pkgs; };
   in ''
     set -x
     mkdir -p "$out/dev-envs" "$out/nix-support" "$out/bin"

pkgs/stdenv/darwin/default.nix

@@ -683,7 +683,7 @@ rec {
       __stdenvImpureHostDeps = commonImpureHostDeps;
       __extraImpureHostDeps = commonImpureHostDeps;
 
-      initialPath = import ../common-path.nix { inherit pkgs; };
+      initialPath = import ../generic/common-path.nix { inherit pkgs; };
       shell = "${pkgs.bash}/bin/bash";
 
       cc = pkgs."${finalLlvmPackages}".libcxxClang;

pkgs/stdenv/linux/default.nix

@@ -397,7 +397,7 @@ in
       preHook = commonPreHook;
 
       initialPath =
-        ((import ../common-path.nix) {pkgs = prevStage;});
+        ((import ../generic/common-path.nix) {pkgs = prevStage;});
 
       extraNativeBuildInputs = [ prevStage.patchelf ] ++
         # Many tarballs come with obsolete config.sub/config.guess that don't recognize aarch64.

pkgs/stdenv/nix/default.nix

@@ -21,7 +21,7 @@ bootStages ++ [
         export NIX_IGNORE_LD_THROUGH_GCC=1
       '';
 
-      initialPath = (import ../common-path.nix) { pkgs = prevStage; };
+      initialPath = (import ../generic/common-path.nix) { pkgs = prevStage; };
 
       cc = import ../../build-support/cc-wrapper {
         inherit lib;