gdb/record_full: add ptid entry for history.

For record full to work on multi-threaded inferiors, we must know to
which thread any given recorded instruction belongs to. This commit adds
this as a new entry for each instruction when an inferior is
multi-threaded. This way, when replaying the instruction, the subsystem
is able to change inferior thread as needed, and avoid SIGILLs when
changing stuff

Ideally, we would only add the PTID entry when we actually need to
switch threads, but that is left as a future improvement.

WIP: replaying doesn't automatically change threads yet, but there was
an attempt