optimize pthread termination in the non-detached case

we can avoid blocking signals by simply using a flag to mark that the
thread has exited and prevent it from getting counted in the rsyscall
signal-pingpong. this restores the original pthread create/join
throughput from before the sigprocmask call was added.

src/internal/pthread_impl.h

@@ -36,6 +36,7 @@ struct pthread {
 	struct __ptcb *cancelbuf;
 	void **tsd;
 	pthread_attr_t attr;
+	volatile int dead;
 };
 
 #define __SU (sizeof(size_t)/sizeof(int))