AArch64: Add stubs for a64 specific helpers

We will need helpers that only make sense with AArch64. Add
helper-a64.{c,h} files as stubs that we can fill with these
helpers in the following patches.

Signed-off-by: Alexander Graf <agraf@suse.de>

From Alexander Graf <agraf@suse.de> aarch64 series 13/60
target-arm: aarch64: add support for 2-src data processing and DIV

this patch adds support for decoding 2-src data processing insns,
and the first users, UDIV and SDIV.

Signed-off-by: Alexander Graf <agraf@suse.de>

[claudio: adapted to new decoder adding the 2-src decoding level,
          always zero-extend result in 32bit mode]

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>

From Alexander Graf <agraf@suse.de> aarch64 series 44/60