x86/spec_ctrl: Merge bti_ist_info and use_shadow_spec_ctrl into spec_ctrl_flags

x86/spec_ctrl: Merge bti_ist_info and use_shadow_spec_ctrl into spec_ctrl_flags

All 3 bits of information here are control flags for the entry/exit code
behaviour.  Treat them as such, rather than having two different variables.

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>
(cherry picked from commit 5262ba2e7799001402dfe139ff944e035dfff928)

Gbp-Pq: Name x86spec_ctrl-merge-bti_ist_info-and-use_.patch