sparc64 expand linear mapping region

sparc64 expand linear mapping region

This prepares for four level page table support in sparc64. It expands
the identity mapped region to 47 bits. It also identifies pieces of
code which are impacted by the changes.

This patch introduces a new config option which isn't available until later
in the patch series.

Signed-off-by: Bob Picco <bob.picco@oracle.com>