[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/cpu/mpc8xx/start.S b/cpu/mpc8xx/start.S
index 33a3f6c..eca4b50 100644
--- a/cpu/mpc8xx/start.S
+++ b/cpu/mpc8xx/start.S
@@ -224,7 +224,7 @@
 /* Alignment exception. */
 	. = 0x600
 Alignment:
-	EXCEPTION_PROLOG
+	EXCEPTION_PROLOG(SRR0, SRR1)
 	mfspr	r4,DAR
 	stw	r4,_DAR(r21)
 	mfspr	r5,DSISR
@@ -242,7 +242,7 @@
 /* Program check exception */
 	. = 0x700
 ProgramCheck:
-	EXCEPTION_PROLOG
+	EXCEPTION_PROLOG(SRR0, SRR1)
 	addi	r3,r1,STACK_FRAME_OVERHEAD
 	li	r20,MSR_KERNEL
 	rlwimi	r20,r23,0,16,16		/* copy EE bit from saved MSR */