xen: sync elfnote.h from xen tree

Sync Xen's elfnote.h header from xen.git to pull in the
XEN_ELFNOTE_PHYS32_RELOC define.

xen commit dfc9fab00378 ("x86/PVH: Support relocatable dom0 kernels")

This is a copy except for the removal of the emacs editor config at the
end of the file.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240823193630.2583107-2-jason.andryuk@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>