spike: Bump version from d1efcdf to 20a2b6d

```
               ========= Summary of gcc testsuite =========
                            | # of unexpected case / # of unique unexpected case
                            |          gcc |          g++ |     gfortran |
     rv64gc/  lp64d/ medlow |    0 /     0 |    0 /     0 |      - |
  rv32imafc/ ilp32f/ medlow |    0 /     0 |    0 /     0 |      - |
```

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>