target/riscv: Consolidate RV32/64 32-bit instructions

target/riscv: Consolidate RV32/64 32-bit instructions

This patch removes the insn32-64.decode decode file and consolidates the
instructions into the general RISC-V insn32.decode decode tree.

This means that all of the instructions are avaliable in both the 32-bit
and 64-bit builds. This also means that we run a check to ensure we are
running a 64-bit softmmu before we execute the 64-bit only instructions.
This allows us to include the 32-bit instructions in the 64-bit build,
while also ensuring that 32-bit only software can not execute the
instructions.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: db709360e2be47d2f9c6483ab973fe4791aefa77.1619234854.git.alistair.francis@wdc.com