cpu: Use the new asBytes function in the protobuf inst tracer.

cpu: Use the new asBytes function in the protobuf inst tracer.

Use this function to get the binary representation of the instruction
rather than referencing the ExtMachInst typed machInst member of the
StaticInst directly. ExtMachInst is an ISA specific type and can't
always be straightforwardly squished into a 32 bit integer.

Change-Id: Ic1f74d6d86eb779016677ae45c022939ce3e2b9f
Reviewed-on: https://gem5-review.googlesource.com/7563
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>