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>

misc: Remove redundant compiler-specific defines

This patch moves away from using M5_ATTR_OVERRIDE and the m5::hashmap
(and similar) abstractions, as these are no longer needed with gcc 4.7
and clang 3.1 as minimum compiler versions.

cpu: Fix InstPBTrace inheritance

This patch fixes an issue that prevented gem5 to be built with C++
config and without Python.

cpu: add support for outputing a protobuf formatted CPU trace

Doesn't support x86 due to static instruction representation.