arm64: entry: Allow handling of undefined instructions from EL1

Rather than panic() when taking an undefined instruction exception from
EL1, allow a hook to be registered in case we want to emulate the
instruction, like we will for the SSBS PSTATE manipulation instructions.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/entry.S

@@ -589,7 +589,7 @@ el1_undef:
 	inherit_daif	pstate=x23, tmp=x2
 	mov	x0, sp
 	bl	do_undefinstr
-	ASM_BUG()
+	kernel_exit 1
 el1_dbg:
 	/*
 	 * Debug exception handling