x86/spec: do not print thunk option selection if not built-in

x86/spec: do not print thunk option selection if not built-in

Since the thunk built-in enable is printed as part of the "Compiled-in
support:" line, avoid printing anything in "Xen settings:" if the thunk is
disabled at build time.

Note the BTI-Thunk option printing is also adjusted to print a colon in the
same way the other options on the line do.

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