go: fix cross-compilation

Because:

* `go-bootstrap` is a native build input of go, so it needs to have
  an offset of -1. Otherwise, e.g. when building a go cross-compiler,
  it will try to make go-bootstrap a cross-compiler too.
* have to specify `buildPackages` for the `stdenv` override, otherwise
  `buildPackages.stdenv` will be the same as `pkgs.gcc8Stdenv`.

pkgs/development/compilers/go/1.14.nix

@@ -13,7 +13,7 @@ let
 
   version = "1.14.15";
 
-  go_bootstrap = callPackage ./bootstrap.nix { };
+  go_bootstrap = buildPackages.callPackage ./bootstrap.nix { };
 
   goBootstrap = runCommand "go-bootstrap" {} ''
     mkdir $out

pkgs/development/compilers/go/1.15.nix

@@ -13,7 +13,7 @@ let
 
   version = "1.15.8";
 
-  go_bootstrap = callPackage ./bootstrap.nix { };
+  go_bootstrap = buildPackages.callPackage ./bootstrap.nix { };
 
   goBootstrap = runCommand "go-bootstrap" {} ''
     mkdir $out

pkgs/development/compilers/go/1.16.nix

@@ -13,7 +13,7 @@ let
 
   version = "1.16";
 
-  go_bootstrap = callPackage ./bootstrap.nix { };
+  go_bootstrap = buildPackages.callPackage ./bootstrap.nix { };
 
   goBootstrap = runCommand "go-bootstrap" {} ''
     mkdir $out

pkgs/development/compilers/go/2-dev.nix

@@ -11,7 +11,7 @@ let
 
   inherit (lib) optionals optionalString;
 
-  go_bootstrap = callPackage ./bootstrap.nix { };
+  go_bootstrap = buildPackages.callPackage ./bootstrap.nix { };
 
   goBootstrap = runCommand "go-bootstrap" {} ''
     mkdir $out