accel/tcg: Add cpu_ld*_code_mmu

accel/tcg: Add cpu_ld*_code_mmu

At least RISC-V has the need to be able to perform a read
using execute permissions, outside of translation.
Add helpers to facilitate this.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Tested-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230325105429.1142530-9-richard.henderson@linaro.org>
Message-Id: <20230412114333.118895-9-richard.henderson@linaro.org>