xtensa/cpu: Make sure cpu_die() doesn't return

cpu_die() doesn't return.  Make that more explicit with a BUG().

BUG() is preferable to unreachable() because BUG() is a more explicit
failure mode and avoids undefined behavior like falling off the edge of
the function into whatever code happens to be next.

Link: https://lore.kernel.org/r/cca346b5c87693499e630291d78fb0bf12c24290.1676358308.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

arch/xtensa/kernel/smp.c

@@ -341,6 +341,8 @@ void __ref cpu_die(void)
 	__asm__ __volatile__(
 			"	movi	a2, cpu_restart\n"
 			"	jx	a2\n");
+
+	BUG();
 }
 
 #endif /* CONFIG_HOTPLUG_CPU */