target/riscv: Implement Ssdbltrp exception handling

target/riscv: Implement Ssdbltrp exception handling

When the Ssdbltrp ISA extension is enabled, if a trap happens in S-mode
while SSTATUS.SDT isn't cleared, generate a double trap exception to
M-mode.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-5-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>