libthread: run first thread in proc on system stack

For pthread systems that are fussy about which stack is used,
this makes sure that threadmain runs on a system stack.
If you only use proccreate (never threadcreate), all threads run
on system stacks.

src/libthread/threadimpl.h

@@ -187,7 +187,6 @@ struct Proc
 };
 
 #define proc() _threadproc()
-#define setproc(p) _threadsetproc(p)
 
 extern Proc *_threadprocs;
 extern Lock _threadprocslock;