Merge changes from topic "feat_state_rework" into integration

* changes:
  feat(fvp): enable FEAT_HCX by default
  refactor(context-mgmt): move FEAT_HCX save/restore into C
  refactor(cpufeat): convert FEAT_HCX to new scheme
  feat(fvp): enable FEAT_FGT by default
  refactor(context-mgmt): move FEAT_FGT save/restore code into C
  refactor(amu): convert FEAT_AMUv1 to new scheme
  refactor(cpufeat): decouple FGT feature detection and build flags
  refactor(cpufeat): check FEAT_FGT in a new way
  refactor(cpufeat): move helpers into .c file, rename FEAT_STATE_
  feat(aarch64): make ID system register reads non-volatile