Retire Thread::TERMINATED

Retire Thread::TERMINATED

Use proper way to detect for thread terimnation instead of
our homegrown flag.

It adds more code than it removes and adds also platform specific
code, neverthless I think is the way to go becuase Thread::TERMINATED
flag is intrinsecly racy given that when we raise it thread is still
_not_ terminated nor it can be, and also we don't want to reinvent
the (broken) wheel of thread termination detection when there is
already available the proper one.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>