CORE-V: Remove versioning support for XCV Extensions

[NandniJamnadas]

* `bfd/elfxx-riscv.c`: Implement default versioning for `XCV` as `1.0`.
* `gas/testsuite/gas/riscv`: Updated all `XCV` tests to remove explicit
  call of versioning in march string.
* `ld/testsuite/gas/riscv`: Updated all `XCV` tests to remove explicit
  call of versioning in march string.

[NandniJamnadas]

GNU Assembler Results:

| Category             | Previous | With commit | Delta |
| -------------------: | -------: | ----------: | ----: |
| Expected passes      |     1256 |        1256 |     - |
| Unexpected failures  |        - |           - |     - | 
| Unexpected successes |        - |           - |     - | 
| Expected failures    |       25 |          25 |     - | 
| Unresolved testcases |        - |           - |     - | 
| Unsupported tests    |        9 |           9 |     - |

GNU Linker Results:

| Category             | Previous | With commit | Delta |
| -------------------: | -------: | ----------: | ----: |
| Expected passes      |      441 |         441 |     - |
| Unexpected failures  |        - |           - |     - | 
| Unexpected successes |        - |           - |     - | 
| Expected failures    |       14 |          14 |     - | 
| Untested testcases   |       26 |          26 |     - |
| Unresolved testcases |        - |           - |     - | 
| Unsupported tests    |      201 |         201 |     - |

No change in test results as I have modified all CORE-V tests to remove explicit call for versions. The results show that you can now call all XCV extensions without explicit version.

[simonpcook]

I think the commit message is missing a reference to the linker test suite directory, but looking at the patch it looks good to me. As we're in the middle of a build, could you hold off merging this for a couple of hours so that they will all be in sync/at the same commit?

[jeremybennett]

Good to merge following review by @simonpcook