mem: Add HTM fields to Request

mem: Add HTM fields to Request

This starts the support of Hardware Transactional Memory on the mem side

* The following flags have been added:

HTM_START: The request starts a HTM transaction
HTM_COMMIT: The request commits a HTM transaction
HTM_CANCEL: The request cancels a HTM transaction
HTM_ABORT: The request aborts a HTM transaction

* The following fields have been added:

_instCount: The instruction count at the time this request is created
_htmAbortCause: The cause for HTM transaction abort

https://gem5.atlassian.net/browse/GEM5-587

Change-Id: Ic582a6566fdd23f30eb92723e629d0c4d4ca10e5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30316
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>