Merge 3 runtime zlibs to 1

There were 3 zlibs laying around in case of linux:
  - 1 used by gcc (for JAR handling in the Java frontend),
  - 1 used by stdenv other tools (e.g. binutils),
  - 1 for the rest of the system.

This was because of some knot-tying issues in the stage/round logic in
the linux specific pure stdenv builder.