stdenv: linux: inherit libxcrypt at xgcc/stage2

libxcrypt was built twice (xgcc and stage2). Adding it to the inherit list
at xgcc and stage2 chains it through: stage1 produces the single build,
xgcc/stage2/stage3 inherit. Total bootstrap-stage builds: 2 -> 1.

libxcrypt is build-time-only — it is needed at build time by tools that
support libcrypt-style password hashing, but its `out` is not in the final
stdenv runtime closure (glibc no longer depends on libxcrypt at runtime;
applications that need it link directly).

Safe with respect to the `disallowedRequisites` check on the final
stdenv-linux.

pkgs/stdenv/linux/default.nix

@@ -344,6 +344,7 @@ in
           perl
           patchelf
           nukeReferences
+          libxcrypt
           ;
         ${localSystem.libc} = prevStage.${localSystem.libc};
         gmp = super.gmp.override { cxx = false; };
@@ -460,6 +461,7 @@ in
           which
           nukeReferences
           autoconf269
+          libxcrypt
           ;
 
         # Avoids infinite recursion, as this is in the build-time dependencies of libc.