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.

mem: Add PacketInfo to be used for packet probe points

This patch fixes a use-after-delete issue in the packet probe points
by adding a PacketInfo struct to retain the key fields before passing
the packet onwards. We want to probe the packet after it is
successfully sent, but by that time the fields may be modified, and
the packet may even be deleted.

Amazingly enough the issue has gone undetected for months, and only
recently popped up in our regressions.

mem: Redesign the stack distance calculator as a probe

This changeset removes the stack distance calculator hooks from the
CommMonitor class and implements a stack distance calculator as a
memory system probe instead. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.