x86: put initial_pg_tables into .bss

Impact: makes vmlinux section information more useful

Don't use ram after _end blindly for pagetables. aka init pages is before _end
put those pg table into .bss

[Adapted to use brk segment - Jeremy]

v2: keep initial page table up to 512M only.
v4: put initial page tables just before _end

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>