Arm backend: mypy fixes (#8092)

Fix mypy warnings in arm_vela.py

In arm_vela.py, function vela_compile() there is a reuse of the
variable block_length that was of different type between uses and mypy
complained.

Change name of block_length that is used inside the loop to
block_length_bytes since its not a int.

Fix mypy warnings in arm_model_evaluator.py

Annotate a dict to fix mypy warning.