x86/spec-ctrl: Rework ibpb_calculations()

x86/spec-ctrl: Rework ibpb_calculations()

... in order to make the SRSO mitigations easier to integrate.

 * Check for AMD/Hygon CPUs directly, rather than assuming based on IBPB.
   In particular, Xen supports synthesising the IBPB bit to guests on Intel to
   allow IBPB while dissuading the use of (legacy) IBRS.
 * Collect def_ibpb_entry rather than opencoding the BTC_NO calculation for
   both opt_ibpb_entry_{pv,hvm}.

No functional change.

This is part of XSA-434 / CVE-2023-20569

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