target/riscv: Fix ssamoswap error handling

target/riscv: Fix ssamoswap error handling

Follow the RISC-V CFI v1.0 spec [1] to fix the exception type
when ssamoswap is disabled by xSSE.

[1] RISC-V CFI spec v1.0, ch2.7 Atomic Swap from a Shadow Stack Location

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250924074818.230010-4-jim.shu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>