target/riscv: implement zicfiss instructions

zicfiss has following instructions
- sspopchk: pops a value from shadow stack and compares with x1/x5.
If they dont match, reports a sw check excepti

target/riscv: implement zicfiss instructions

zicfiss has following instructions
- sspopchk: pops a value from shadow stack and compares with x1/x5.
If they dont match, reports a sw check exception with tval = 3.
- sspush: pushes value in x1/x5 on shadow stack
- ssrdp: reads current shadow stack
- ssamoswap: swaps contents of shadow stack atomically

sspopchk/sspush/ssrdp default to zimop if zimop implemented and SSE=0

If SSE=0, ssamoswap is illegal instruction exception.

This patch implements shadow stack operations for qemu-user and shadow
stack is not protected.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Co-developed-by: Jim Shu <jim.shu@sifive.com>
Co-developed-by: Andy Chiu <andy.chiu@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241008225010.1861630-17-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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