stdenv: force gmp to rebuild in stage4 of the bootstrap

As explained in the comment, this ensures that stage4-coreutils does
not leak a reference to the bootstrap-files by way of libgmp.  This
will allow the next patch in this series to build stage4-coreutils
using a dynamically-linked (rather than statically-linked) libgmp.

pkgs/stdenv/linux/default.nix

@@ -332,7 +332,7 @@ in
       # because gcc (since JAR support) already depends on zlib, and
       # then if we already have a zlib we want to use that for the
       # other purposes (binutils and top-level pkgs) too.
-      inherit (prevStage) gettext gnum4 bison gmp perl texinfo zlib linuxHeaders libidn2 libunistring;
+      inherit (prevStage) gettext gnum4 bison perl texinfo zlib linuxHeaders libidn2 libunistring;
       ${localSystem.libc} = getLibc prevStage;
       binutils = super.binutils.override {
         # Don't use stdenv's shell but our own
@@ -343,6 +343,11 @@ in
         };
       };
 
+      # force gmp to rebuild so we have the option of dynamically linking
+      # libgmp without creating a reference path from:
+      #   stage5.gcc -> stage4.coreutils -> stage3.glibc -> bootstrap
+      gmp = super.gmp.override { stdenv = self.stdenv; };
+
       gcc = lib.makeOverridable (import ../../build-support/cc-wrapper) {
         nativeTools = false;
         nativeLibc = false;
@@ -410,7 +415,7 @@ in
         # Simple executable tools
         concatMap (p: [ (getBin p) (getLib p) ]) [
             gzip bzip2 xz bash binutils.bintools coreutils diffutils findutils
-            gawk gnumake gnused gnutar gnugrep gnupatch patchelf ed
+            gawk gmp gnumake gnused gnutar gnugrep gnupatch patchelf ed
           ]
         # Library dependencies
         ++ map getLib (