Add macro for shadow stack as well

Author: kito-cheng

src/c-api.adoc

@@ -32,6 +32,7 @@ a|
 | `+__riscv_landing_pad+` | 1 | Defined if the landing pad is enabled.
 | `+__riscv_landing_pad_unlabeled+` | 1 | Defined if the unlabeled labeling scheme is enabled.
 | `+__riscv_landing_pad_func_sig+` | 1 | Defined if the function signature based labeling scheme is enabled.
+| `+__riscv_shadow_stack+` | 1 | Defined if the shadow stack is enabled.
 |===
 
 [id=__riscv_v_min_vlen]