cpu: Allow storing an invalid HTM checkpoint

cpu: Allow storing an invalid HTM checkpoint

Commits 02745afd and f9b4e32 introduced a mechanism for creating checkpoint
objects for hardware transactional memory (HTM) and Arm TME. Because the
checkpoint object also contains the local UID of a transaction, it is
needed before any architectural checkpointing takes places. This caused
segfaults when running HTM codes.

This commit allows ISAs to allocate a checkpoint once at the beginning
of simulation.  In order to do that we need to remove the validity check
assertion; the cpt will become valid only after a first successfull
transaction start

Change-Id: I233d01805f8ab655131ed8cd6404950a2bf6fbc7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35015
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>