glibc: Split into multiple outputs

This reduces the size of closures a lot by getting rid of Glibc's
header files, static libraries and utilities.  For instance, the
closure of GNU Hello went from 41 MiB to 22 MiB.

TODO: We may want to merge back in parts of Glibc that we previously
split off (such as the info files).

pkgs/stdenv/linux/default.nix

@@ -7,7 +7,7 @@
 # The function defaults are for easy testing.
 { system ? builtins.currentSystem
 , allPackages ? import ../../top-level/all-packages.nix
-, platform ? null, config }:
+, platform ? null, config ? {} }:
 
 rec {