target/riscv: fixes a bug against `ssamoswap` behavior in M-mode

target/riscv: fixes a bug against `ssamoswap` behavior in M-mode

Commit f06bfe3dc38c ("target/riscv: implement zicfiss instructions") adds
`ssamoswap` instruction. `ssamoswap` takes the code-point from existing
reserved encoding (and not a zimop like other shadow stack instructions).
If shadow stack is not enabled (via xenvcfg.SSE) and effective priv is
less than M then `ssamoswap` must result in an illegal instruction
exception. However if effective priv is M, then `ssamoswap` results in
store/AMO access fault. See Section "22.2.3. Shadow Stack Memory
Protection" of priv spec.

Fixes: f06bfe3dc38c ("target/riscv: implement zicfiss instructions")
Reported-by: Ved Shanbhogue <ved@rivosinc.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250306064636.452396-2-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit d2c5759c8dd4c00195d4ebecc7d009e41df6baef)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>