add stubs for other systems.

add threadspawn.

src/libthread/PowerMacintosh.c

@@ -25,3 +25,15 @@ _threadinitstack(Thread *t, void (*f)(void*), void *arg)
 	t->sched.sp = (ulong)tos-80;
 }
 
+void
+_threadinswitch(int enter)
+{
+	USED(enter);
+}
+
+void
+_threadstacklimit(void *addr)
+{
+	USED(addr);
+}
+

src/libthread/exec-unix.c

@@ -115,7 +115,8 @@ threadexecl(Channel *pidc, int fd[3], char *f, ...)
 	args[n] = 0;
 	va_end(arg);
 
-	_threadexec(pidc, fd, f, args, 1);
+	if(_threadexec(pidc, fd, f, args, 1) >= 0)
+		threadexits(nil);
 }
 
 static void

src/libthread/sun4u.c

@@ -39,3 +39,15 @@ _threadinitstack(Thread *t, void (*f)(void*), void *arg)
 	if(0) print("tis %x %x at %x\n", f, arg, t->sched.sp);
 }
 
+void
+_threadinswitch(int enter)
+{
+	USED(enter);
+}
+
+void
+_threadstacklimit(void *addr)
+{
+	USED(addr);
+}
+