target/riscv: Implement dynamic establishment of custom decoder

target/riscv: Implement dynamic establishment of custom decoder

In this patch, we modify the decoder to be a freely composable data
structure instead of a hardcoded one. It can be dynamically builded up
according to the extensions.
This approach has several benefits:
1. Provides support for heterogeneous cpu architectures. As we add decoder in
   RISCVCPU, each cpu can have their own decoder, and the decoders can be
   different due to cpu's features.
2. Improve the decoding efficiency. We run the guard_func to see if the decoder
   can be added to the dynamic_decoder when building up the decoder. Therefore,
   there is no need to run the guard_func when decoding each instruction. It can
   improve the decoding efficiency
3. For vendor or dynamic cpus, it allows them to customize their own decoder
   functions to improve decoding efficiency, especially when vendor-defined
   instruction sets increase. Because of dynamic building up, it can skip the other
   decoder guard functions when decoding.
4. Pre patch for allowing adding a vendor decoder before decode_insn32() with minimal
   overhead for users that don't need this particular vendor decoder.

Signed-off-by: Huang Tao <eric.huang@linux.alibaba.com>
Suggested-by: Christoph Muellner <christoph.muellner@vrull.eu>
Co-authored-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240506023607.29544-1-eric.huang@linux.alibaba.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>