target/arm: Create decodetree skeleton for A64

target/arm: Create decodetree skeleton for A64

The A64 translator uses a hand-written decoder for everything except
SVE or SME.  It's fairly well structured, but it's becoming obvious
that it's still more painful to add instructions to than the A32
translator, because putting a new instruction into the right place in
a hand-written decoder is much harder than adding new instruction
patterns to a decodetree file.

As the first step in conversion to decodetree, create the skeleton of
the decodetree decoder; where it does not handle instructions we will
fall back to the legacy decoder (which will be for everything at the
moment, since there are no patterns in a64.decode).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230512144106.3608981-3-peter.maydell@linaro.org