s390/boot: Swap vmalloc and Lowcore/Real Memory Copy areas

This is a preparatory rework to allow uncoupling virtual
and physical addresses spaces.

Currently the order of virtual memory areas is (the lowcore
and .amode31 section are skipped, as it is irrelevant):

	identity mapping (the kernel is contained within)
	vmemmap
	vmalloc
	modules
	Absolute Lowcore
	Real Memory Copy

In the future the kernel will be mapped separately and placed
to the end of the virtual address space, so the layout would
turn like this:

	identity mapping
	vmemmap
	vmalloc
	modules
	Absolute Lowcore
	Real Memory Copy
	kernel

However, the distance between kernel and modules needs to be as
little as possible, ideally - none. Thus, the Absolute Lowcore
and Real Memory Copy areas would stay in the way and therefore
need to be moved as well:

	identity mapping
	vmemmap
	Absolute Lowcore
	Real Memory Copy
	vmalloc
	modules
	kernel

To facilitate such layout swap the vmalloc and Absolute Lowcore
together with Real Memory Copy areas. As result, the current
layout turns into:

	identity mapping (the kernel is contained within)
	vmemmap
	Absolute Lowcore
	Real Memory Copy
	vmalloc
	modules

This will allow to locate the kernel directly next to the
modules once it gets mapped separately.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>