x86/spec_ctrl: Express Xen's choice of MSR_SPEC_CTRL value as a variable

x86/spec_ctrl: Express Xen's choice of MSR_SPEC_CTRL value as a variable

At the moment, we have two different encodings of Xen's MSR_SPEC_CTRL value,
which is a side effect of how the Spectre series developed.  One encoding is
via an alias with the bottom bit of bti_ist_info, and can encode IBRS or not,
but not other configurations such as STIBP.

Break Xen's value out into a separate variable (in the top of stack block for
XPTI reasons) and use this instead of bti_ist_info in the IST path.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>