From: Brian Gerst <bgerst@didntduck.org>

- Move empty_zero_page and swapper_pg_dir to BSS.  This requires that BSS
  is cleared earlier, but reclaims over 3k that was lost due to page
  alignment.

- Move stack_start, ready, and int_msg, boot_gdt_descr, idt_descr, and
  cpu_gdt_descr to .data.  They were interfering with disassembly while in
  .text.