Handle SIGABRT a little more nicely.

Handle SIGABRT a little more nicely.

base/misc.cc:
    Don't dump trace in panic(), SIGABRT handler will do it now.
sim/main.cc:
    Add SIGABRT handler that prints curTick and dumps buffered trace (if any).
    This doesn't work as well as I would like since the buffered trace records
    often contain stale references to stack-resident temporary std::string objects.
    Someday we'll have to put in a fix for that.

--HG--
extra : convert_revision : 67576efbf5c10e63e255fc9a9ec520326fd3567b