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

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

When the Ssdbltrp extension is enabled, SSTATUS.SDT field is cleared
when executing sret. When executing mret/mnret, SSTATUS.SDT is cleared
when returning to U, VS or VU and VSSTATUS.SDT is cleared when returning
to VU from HS.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-4-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>