x86: partially revert use of 2M mappings for hypervisor image

x86: partially revert use of 2M mappings for hypervisor image

As explained by Andrew in
http://lists.xenproject.org/archives/html/xen-devel/2016-03/msg01380.html
that change makes the uncompressed xen.gz image too large for certain
boot environments. As a result this change makes some of the effects of
commits cf393624ee ("x86: use 2M superpages for text/data/bss
mappings") and 53aa3dde17 ("x86: unilaterally remove .init mappings")
conditional, restoring alternative previous code where necessary. This
is so that xen.efi can still benefit from the new mechanisms, as it is
unaffected by said limitations.

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