make the backtrace more easy to read

Demangle C++ symbols so that the output is human readable.

The original backtrace looks like this:

 ./perftest(_ZN5mongo12QueryPlanSetC1EPKcSt8auto_ptrINS_17FieldRangeSetPairEES5_RKNS_7BSONObjES8_bPKNS_11BSONElementEbS8_S8_bb+0x214) [0x8222fc4]

With the patch, the backtrace looks like:

 ./perftest(mongo::QueryPlanSet::QueryPlanSet(char const*, std::auto_ptr<mongo::FieldRangeSetPair>, std::auto_ptr<mongo::FieldRangeSetPair>, mongo::BSONObj const&, mongo::BSONObj const&, bool, mongo::BSONElement const*, bool, mongo::BSONObj const&, mongo::BSONObj const&, bool, bool)+0x214) [0x8223464]

References:
 http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt12ch39.html