x86: construct the {l2,l3}_bootmap at compile time

x86: construct the {l2,l3}_bootmap at compile time

... rather than at runtime.

The bootmaps are discarded in zap_low_mappings(), so the tables themselves can
live in .init.data and be reclaimed after boot.

Hooking the l1_identmap into l2_xenmap stays for safety, along with a longer
comment explaining why.

This does not change the EFI construction of {l2,l3}_bootmap.  EFI already
constructs them cleanly in their relocated form.

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