Implement FUCKWIT for arm64; unmap the kernel almost entirely while userland

is running.  This provides protection against meltown on cores that are
vilnerable (just Cortex-A75 so far) but also seems to be an essential to
protect against spectre-like attacks against the kernel.

This implementation only exposes a single treampoline page that does not
contain any kernel virtual addresses and also hides the real virtual address
of the exception vectors, which helps on cores vulnerable to "variant 3a"
(Cortex-A57, Cortex-A72).  The implementation is inspired by the work done
by Will Deacon for Linux, but there are no knobs to turn it off.  The
overhead is fairly limited: around 3-4% slowdown on Cortex-A57.

ok patrick@, deraadt@

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