x86/kexec: Move relocate_kernel to kernel .data section

Now that the copy is executed instead of the original, the relocate_kernel
page can live in the kernel's .text section. This will allow subsequent
commits to actually add real data to it and clean up the code somewhat as
well as making the control page ROX.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Dave Young <dyoung@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Link: https://lore.kernel.org/r/20241205153343.3275139-9-dwmw2@infradead.org