x86: support data operand independent timing mode

[1] specifies a long list of instructions which are intended to exhibit
timing behavior independent of the data they operate on. On certain
hardware this independence is optional, controlled by a bit in a new
MSR. Provide a command line option to control the mode Xen and its
guests are to operate in, with a build time control over the default.
Longer term we may want to allow guests to control this.

Since Arm64 supposedly also has such a control, put command line option
and Kconfig control in common files.

[1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/best-practices/data-operand-independent-timing-isa-guidance.html

Requested-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

CHANGELOG.md

@@ -33,6 +33,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
    nodes using a device tree overlay binary (.dtbo).
  - Introduce two new hypercalls to map the vCPU runstate and time areas by
    physical rather than linear/virtual addresses.
+ - On x86, support for enforcing system-wide operation in Data Operand
+   Independent Timing Mode.
 
 ### Removed
  - On x86, the "pku" command line option has been removed.  It has never