target/riscv: Add a REQUIRE_32BIT macro

target/riscv: Add a REQUIRE_32BIT macro

With the changes to Zb[abcs], there's some encodings that are
different in RV64 and RV32 (e.g., for rev8 and zext.h). For these,
we'll need a helper macro allowing us to select on RV32, as well.

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>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 20210911140016.834071-13-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>