cpu: Move the softmmu tlb to CPUNegativeOffsetState

cpu: Move the softmmu tlb to CPUNegativeOffsetState

We have for some time had code within the tcg backends to
handle large positive offsets from env.  This move makes
sure that need not happen.  Indeed, we are able to assert
at build time that simple offsets suffice for all hosts.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>