cpu: Add HTM Instruction Flags

cpu: Add HTM Instruction Flags

IsHtmStart: Starts a HTM transaction
IsHtmStop: Stops (commits) a HTM transaction
IsHtmCancel: Explicitely aborts a HTM transaction

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

Change-Id: I33144f97a2009e28b0c64777f0313cd6eadb7ff9
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30321
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>