scons: Identify runs that fail and runs with stats differences

scons: Identify runs that fail and runs with stats differences

This patch changes the regression script such that it is possible to
identify the runs that fail with an exit code, and those that finish
with stats differences. The ones that truly fail are reported as
FAILED, and those that finish with changed stats as CHANGED.

The yellow colour has been reclaimed from the skipped regressions and
is now used for the changed ones. With no obvious good option left the
skipped ones are now in cyan.

While I was editing the script I also bumped any occurence of M5 to
gem5.