target/riscv: Implement Smdbltrp sret, mret and mnret behavior

When the Ssdbltrp extension is enabled, SSTATUS.MDT field is cleared
when executing sret if executed in M-mode. When executing mret/mnret,
SSTATUS.MDT is cleared.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-8-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>