os: Simplify os::thread_specific_ptr.
os: Simplify os::thread_specific_ptr.

Instead of mimicking boost::thread_specific_ptr, do something that allows
to mimick more closely compiler TLS support.

schulmar/apitrace,trtt/apitrace,swq0553/apitrace,swq0553/apitrace,trtt/apitrace,schulmar/apitrace,PeterLValve/apitrace,PeterLValve/apitrace,joshua5201/apitrace,swq0553/apitrace,tuanthng/apitrace,EoD/apitrace,joshua5201/apitrace,surround-io/apitrace,tuanthng/apitrace,EoD/apitrace,schulmar/apitrace,trtt/apitrace,schulmar/apitrace,tuanthng/apitrace,joshua5201/apitrace,EoD/apitrace,EoD/apitrace,surround-io/apitrace,apitrace/apitrace,PeterLValve/apitrace,apitrace/apitrace,apitrace/apitrace,surround-io/apitrace,schulmar/apitrace,tuanthng/apitrace,swq0553/apitrace,surround-io/apitrace,joshua5201/apitrace,joshua5201/apitrace,trtt/apitrace,surround-io/apitrace,swq0553/apitrace,apitrace/apitrace,EoD/apitrace,tuanthng/apitrace,trtt/apitrace,PeterLValve/apitrace