target/arm/tcg: Don't build AArch64 decodetree files for qemu-system-arm

target/arm/tcg: Don't build AArch64 decodetree files for qemu-system-arm

Currently we list all the Arm decodetree files together and add them
unconditionally to arm_ss.  This means we build them for both
qemu-system-aarch64 and qemu-system-arm.  However, some of them are
AArch64-specific, so there is no need to build them for
qemu-system-arm.  (Meson is smart enough to notice that the generated
.c.inc file is not used by any objects that go into qemu-system-arm,
so we only unnecessarily run decodetree, not anything more
heavyweight like a recompile or relink, but it's still unnecessary
work.)

Split gen into gen_a32 and gen_a64, and only add gen_a64 for
TARGET_AARCH64 compiles.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20230718104628.1137734-1-peter.maydell@linaro.org