alpha/cpu: Make sure arch_cpu_idle_dead() doesn't return

arch_cpu_idle_dead() 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/19ffef09a175fecb783abcd44d6bcfeade2857eb.1676358308.git.jpoimboe@kernel.orgSigned-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

arch/alpha/kernel/process.c

@@ -63,6 +63,7 @@ void arch_cpu_idle(void)
 void arch_cpu_idle_dead(void)
 {
 	wtint(INT_MAX);
+	BUG();
 }
 #endif /* ALPHA_WTINT */