target/riscv: Add check for supported privilege mode combinations

There are 3 suggested privilege mode combinations listed in section 1.2
of the riscv-privileged spec(draft-20220717):
1) M, 2) M, U 3) M, S, U

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-Id: <20220718130955.11899-2-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -721,6 +721,12 @@ static void riscv_cpu_realize(DeviceState *dev, Error **errp)
             return;
         }
 
+        if (cpu->cfg.ext_s && !cpu->cfg.ext_u) {
+            error_setg(errp,
+                       "Setting S extension without U extension is illegal");
+            return;
+        }
+
         if (cpu->cfg.ext_f && !cpu->cfg.ext_icsr) {
             error_setg(errp, "F extension requires Zicsr");
             return;