arm64: simplify expression

!a != !b -> a != b; this part was lifted from NetBSD, and I clearly did
not reconsider that these are bools (I'm guessing they were ints in an
earlier iteration of the NetBSD implementation).

while we're here, it should be easy to see that we've covered all of the
cases but let's add in an __assert_unreachable() to make it easier on
the eyes.

Reported by: jrtc27