[ppc] Fix build breakage for all non-4xx PowerPC variants.

- adapt to the more generic EXCEPTION_PROLOG and CRIT_EXCEPTION macros
- minor 4xx cleanup
diff --git a/include/mpc83xx.h b/include/mpc83xx.h
index 60fc214..cbf41c3 100644
--- a/include/mpc83xx.h
+++ b/include/mpc83xx.h
@@ -25,6 +25,7 @@
 /* System reset offset (PowerPC standard)
  */
 #define EXC_OFF_SYS_RESET		0x0100
+#define	_START_OFFSET			EXC_OFF_SYS_RESET
 
 /* IMMRBAR - Internal Memory Register Base Address
  */