target/riscv: Use REQUIRE_64BIT in amo_check64

target/riscv: Use REQUIRE_64BIT in amo_check64

Use the same REQUIRE_64BIT check that we use elsewhere,
rather than open-coding the use of is_32bit.

Reviewed-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20211020031709.359469-7-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>