x86: switch default mapping attributes to non-executable

x86: switch default mapping attributes to non-executable

Only a very limited subset of mappings need to be done as executable
ones; in particular the direct mapping should not be executable to
limit the damage attackers can cause by exploiting security relevant
bugs.

The EFI change at once includes an adjustment to set NX only when
supported by the hardware.

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