[RISCV] Improve error message when the extension is not supported (#83989)

If the "march" has some extension with version that is not supported, it
returns the error message like: "error: invalid arch name 'some_arch',
unsupported version number 2.0 for extension 'some_arch'", which is not
precise enough, it should return the message that only tells users "the
extension is not supported".