Commit 28b6fb6 broke cross compilation, since it changed lots of packages to refer to stdenv.ccCross, which does not exit. Only gccCross does.
Grepping for gccCross reveals a lot of definitions and uses which may not all be related to the stdenv meaning, so I'm not sure what the fix is. Maybe just rename all gccCross to ccCross?

Simple test case:

[ambro@nixos:/etc/nixos/nixpkgs/pkgs/stdenv/linux]$ nix-build make-bootstrap-tools-crosspi.nix -A build -o ~/bootstrap-tools
error: attribute ‘ccCross.libc’ missing, at "/etc/nixos/nixpkgs/pkgs/tools/misc/coreutils/default.nix":30:22
(use ‘--show-trace’ to show detailed location information)