Some background to help understand. We have following frame enumerations for RISC-V:

```
enum {
  link_offset = -2,
  return_addr_offset = -1,
  sender_sp_offset = 0
};
```
The values are compatible with the platform ABI and are different from the other platforms like x64 or aarch64. Especially, `sender_sp_offset` is 0 for RISC-V compared to 2 for the other platforms. As a result, there exists some differences in places where code uses value of `sender_sp_offset` as an sp offset when fp is needed. For RISC-V, we used constant number 2 instead of `sender_sp_offset` as the sp offset. But the code will be more readable if we use `frame::metadata_words` which has the same value.