powerpc/8xx: Fix software emulation interrupt

For unimplemented instructions or unimplemented SPRs, the 8xx triggers
a "Software Emulation Exception" (0x1000). That interrupt doesn't set
reason bits in SRR1 as the "Program Check Exception" does.

Go through emulation_assist_interrupt() to set REASON_ILLEGAL.

Fixes: fbbcc3bb ("powerpc/8xx: Remove SoftwareEmulation()")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ad782af87a222efc79cfb06079b0fd23d4224eaf.1612515180.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_8xx.S

@@ -165,7 +165,7 @@ SystemCall:
 /* On the MPC8xx, this is a software emulation interrupt.  It occurs
  * for all unimplemented and illegal instructions.
  */
-	EXCEPTION(0x1000, SoftEmu, program_check_exception, EXC_XFER_STD)
+	EXCEPTION(0x1000, SoftEmu, emulation_assist_interrupt, EXC_XFER_STD)
 
 	. = 0x1100
 /*