linux-user: Allocate thunk size dynamically

We store all struct types in an array of static size without ever
checking whether we overrun it. Of course some day someone (like me
in another, ancient ALSA enabling patch set) will run into the limit
without realizing it.

So let's make the allocation dynamic. We already know the number of
structs that we want to allocate, so we only need to pass the variable
into the respective piece of code.

Also, to ensure we don't accidently overwrite random memory, add some
asserts to sanity check whether a thunk is actually part of our array.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
