CORE-V: Remove versioning support for XCV Extensions

    * 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.

Signed-off-by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>