One might think of this as follow-on to XSA-451, but that's not quite
the right order of events.

There are a few open aspects; see in particular the final patch.

v4 addresses review feedback along with some significant re-basing that
was necessary due to other recent changes. It includes one new tidying
patch. See individual patches for details.

1: record SSP at non-guest entry points
2: traps: use entry_ssp in fixup_exception_return()
3: traps: reduce indentation in fixup_exception_return()
4: prefer shadow stack for producing call traces

Jan