Add tests to make sure common instructions are accepted in RV64
and not just RV32.
Thanks for extending the test coverage!
Given that other than RVC, the RV64 variants of instruction set extensions are always additive, I have a slight preference for just adding some RV64 RUN lines to the rv32*-valid.s tests rather than copying and pasting the common instructions to the rv64*-valid.s files. This is consistent with what we do for the base extensions.
But this works fine, so I'm happy enough for it to land as is.