x86/alt: Adjust _alternative_instructions() to not create shadow stacks

x86/alt: Adjust _alternative_instructions() to not create shadow stacks

The current alternatives algorithm clears CR0.WP and writes into .text.  This
has a side effect of the mappings becoming shadow stacks once CET is active.

Adjust _alternative_instructions() to clean up after itself.  This involves
extending the set of bits modify_xen_mappings() to include Dirty (and Accessed
for good measure).

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