Move UsePrivateSystemHeaders into arch-specific block.
Fix build of pkgman on x86_64 by use of B_PRI... define.
Fix broken build of package kit (due to warnings) for x86_64.
Drop use of non-existing variable in KernelAddon rule.

* TARGET_KERNEL_PIC_FLAGS was probably meant to be 
  TARGET_KERNEL_PIC_CCFLAGS at the time this had been added in 2005.
  As correcting the name would mean that kernel add-ons would be 
  compiled such that they wouldn't be position independent, dropping
  the variable makes more sense (which is just a cleanup and doesn't 
  change anything due to that variable being always empty)
Fix build problems of btrfs and packagefs for x86_64.

* Both filesystems used to link to a static kernel-zlib, which 
  was being built with -fno-pic. This doesn't work on x86_64 as the 
  filesystem add-ons are meant to be relocatable, which requires their 
  code to be compiled as position independent.
  Solve that by moving zlib into the kernel, so any add-on can just use
  it from there (packagefs is mandatory, so we can't really do without 
  zlib anyway).