x86/spec-ctrl: Consistently halt speculation using int3

x86/spec-ctrl: Consistently halt speculation using int3

The RSB stuffing loop and retpoline thunks date from the very beginning, when
halting speculation was a brand new field.

These days, we've largely settled on int3 for halting speculation in
non-architectural paths.  It's a single byte, and is fully serialising - a
requirement for delivering #BP if it were to execute.

Update the thunks.  Mostly for consistency across the codebase, but it does
shrink every entrypath in Xen by 6 bytes which is a marginal win.

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