stacks: Make sure to initialize Call16Data

Initialize the Call16Data at startup - otherwise some early yield()
calls may check for interrupts without using the preferred A20
setting.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/stacks.c b/src/stacks.c
index ef6a707..9fec2fb 100644
--- a/src/stacks.c
+++ b/src/stacks.c
@@ -496,6 +496,7 @@
 thread_setup(void)
 {
     CanInterrupt = 1;
+    call16_override(1);
     if (! CONFIG_THREADS)
         return;
     ThreadControl = romfile_loadint("etc/threads", 1);