x86, memblock: Add memblock_x86_to_bootmem()

memblock_x86_to_bootmem() will reserve memblock.reserved.region in
bootmem after bootmem is set up.

We can use it to with all arches that support memblock later.

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