target/riscv: iterate over a table of decoders

To split up the decoder into multiple functions (both to support
vendor-specific opcodes in separate files and to simplify maintenance
of orthogonal extensions), this changes decode_op to iterate over a
table of decoders predicated on guard functions.

This commit only adds the new structure and the table, allowing for
the easy addition of additional decoders in the future.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220202005249.3566542-6-philipp.tomsich@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>