Add a preprocessor definitions to indidcate the landing pad labeling scheme

Also add 2 more constant value to let user able to determine the scheme.

Example code:
```c
 #ifdef __riscv_landing_pad
   #if __riscv_landing_pad == __riscv_landing_pad_simple
     ...
   #elif __riscv_landing_pad == __riscv_landing_pad_func_sig
     ...
   #else
     #error "Unknown labeling scheme"
   #endif
 #endif
```

Author: kito-cheng

riscv-c-api.md

@@ -47,6 +47,9 @@ https://creativecommons.org/licenses/by/4.0/.
 | __riscv_misaligned_fast | 1  | Misaligned accesses are fast. |
 | __riscv_misaligned_slow | 1  | Misaligned accesses are supported, but may be substantially slower than aligned accesses. |
 | __riscv_misaligned_avoid | 1  | Misaligned accesses are not supported and could trap. (see [ __riscv_misaligned_{fast,slow,avoid}](#__riscv_misaligned_{fast,slow,avoid}) |
+| __riscv_landing_pad | <N> (See [__riscv_landing_pad](#__riscv_landing_pad) | Defined if the landing pad is enabled, and specifies which labeling scheme is used. |
+| __riscv_landing_pad_simple | 1 | Constant value for the simple labeling scheme. |
+| __riscv_landing_pad_func_sig | 2 | Constant value for the function signature based labeling scheme. |
 
 ### __riscv_v_min_vlen
 
@@ -107,6 +110,10 @@ and avoid to -mstrict-align, if specified.
 Perhaps obvious, but these are mutually exclusive, so only one is defined at a time
 for a compilation unit.
 
+### __riscv_landing_pad
+
+The value of `__riscv_landing_pad` indicates which labeling scheme is used. It
+can be either `__riscv_landing_pad_simple` or `__riscv_landing_pad_func_sig`.
 
 ### Architecture Extension Test Macros