make-bootstrap-tools: grab libc from stdenv.cc.libc

This is the same in current cases AFAICT,
other than uses musl instead of glibc when musl-native.

pkgs/stdenv/linux/make-bootstrap-tools.nix

···
 
 let
   pkgs = import ../../.. { inherit localSystem crossSystem; };
-  libc = if pkgs.hostPlatform != pkgs.buildPlatform
-          then pkgs.libcCross
-          else pkgs.glibc;
+  libc = pkgs.stdenv.cc.libc;
 in with pkgs; rec {