target/riscv: introduce ssp and enabling controls for zicfiss

zicfiss introduces a new state ssp ("shadow stack register") in cpu.
ssp is expressed as a new unprivileged csr (CSR_SSP=0x11) and holds
virtual address for shadow stack as programmed by software.

Shadow stack (for each mode) is enabled via bit3 in *envcfg CSRs.
Shadow stack can be enabled for a mode only if it's higher privileged
mode had it enabled for itself. M mode doesn't need enabling control,
it's always available if extension is available on cpu.

This patch also implements helper bcfi function which determines if bcfi
is enabled at current privilege or not.

Adds ssp to migration state as well.

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: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241008225010.1861630-12-debug@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -1014,6 +1014,8 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
 
     /* on reset elp is clear */
     env->elp = false;
+    /* on reset ssp is set to 0 */
+    env->ssp = 0;
 
     env->xl = riscv_cpu_mxl(env);
     riscv_cpu_update_mask(env);