target/arm: Add stubs for AArch32 Neon decodetree

target/arm: Add stubs for AArch32 Neon decodetree

Add the infrastructure for building and invoking a decodetree decoder
for the AArch32 Neon encodings.  At the moment the new decoder covers
nothing, so we always fall back to the existing hand-written decode.

We follow the same pattern we did for the VFP decodetree conversion
(commit 78e138bc1f672c145ef6ace74617d and following): code that deals
with Neon will be moving gradually out to translate-neon.vfp.inc,
which we #include into translate.c.

In order to share the decode files between A32 and T32, we
split Neon into 3 parts:
 * data-processing
 * load-store
 * 'shared' encodings

The first two groups of instructions have similar but not identical
A32 and T32 encodings, so we need to manually transform the T32
encoding into the A32 one before calling the decoder; the third group
covers the Neon instructions which are identical in A32 and T32.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200430181003.21682-4-peter.maydell@linaro.org