RISC-V: Don't report the mismatched version warning for the implicit extensions.

bfd/
    * elfnn-riscv.c (riscv_version_mismatch): Do not report the warning
    when the version of input or output is RISCV_UNKNOWN_VERSION, since
    the extension is added implicitly.
    * elfxx-riscv.c: Updated the obsolete comments.
    (RISCV_UNKNOWN_VERSION): Moved to elfxx-riscv.h.
    * elfxx-riscv.h (RISCV_UNKNOWN_VERSION): Added.