x86/boot: Create the l2_xenmap[] mappings dynamically

x86/boot: Create the l2_xenmap[] mappings dynamically

The build-time construction of l2_xenmap[] imposes an arbitrary limit of 16M
total, which is a limit looking to be lifted.

Adjust both the BIOS and EFI paths to fill it in dynamically, based on the
final linked size of Xen.  l2_xenmap[] stays between __page_tables_{start,end}
(rather than move into .bss.page_aligned) as it is expected to gain a
different pagetable reference shortly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>