x86/spec-ctrl: Split the "Hardware features" diagnostic line

x86/spec-ctrl: Split the "Hardware features" diagnostic line

Separate the read-only hints from the features requiring active actions on
Xen's behalf.

Also take the opportunity split the IBRS/IBPB and IBPB mess.  More features
with overlapping enumeration are on the way, and and it is not useful to split
them like this.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 565ebcda976c05b0c6191510d5e32b621a2b1867
master date: 2021-09-08 14:16:19 +0100