Revert "Fix build problems of btrfs and packagefs for x86_64."

This reverts commit f272e63f393449d3a816970ca632764895a78cd6.
2nd attempt at fixing build of btrfs and packagefs.

* adding zlib to the kernel unfortunately introduces a cyclic dependency
  with respect to the zlib, haiku and haiku_devel packages (AFAICS)
* circumvent this by building kernel_zlib as a static library again,
  this time with PIC, such that it can be used by kernel add-ons
Handle rigged source packages when determining port name.

* I'm not sure this makes much of a difference, but I thought it's 
  better to play it safe.