Reorganize i386/kernel/head.S to eliminate the .org directives.
- boot_pgtables move to .data.init so they can be reclaimed after the 
final pagetables are set up.
- empty_zero_page and swapper_pg_dir move to .data.page_aligned
- the IDT and GDT descriptors are moved to .data

The end effect is that we can now fully use the first page of head.S 
saving about half a page of memory.

--
				Brian Gerst