target/riscv: Reorg sum check in get_physical_address

target/riscv: Reorg sum check in get_physical_address

Implement this by adjusting prot, which reduces the set of
checks required.  This prevents exec to be set for U pages
in MMUIdx_S_SUM.  While it had been technically incorrect,
it did not manifest as a bug, because we will never attempt
to execute from MMUIdx_S_SUM.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-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: <20230325105429.1142530-26-richard.henderson@linaro.org>
Message-Id: <20230412114333.118895-26-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>