nmi: Don't try to switch onto extra stack in NMI handler

The NMI could occur when already on the extra stack, which would
corrupt it.  Always use the current stack on an NMI to avoid this.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/romlayout.S b/src/romlayout.S
index fedadfe..53cc0f5 100644
--- a/src/romlayout.S
+++ b/src/romlayout.S
@@ -548,7 +548,10 @@
         ENTRY_INTO32 _cfunc32flat_handle_post   // Normal entry point
         ORG 0xe2c3
-        IRQ_ENTRY 02
+        .global entry_02
+        ENTRY handle_02  // NMI handler does not switch onto extra stack
+        iretw
         ORG 0xe3fe
         .global entry_13_official