RISC-V: Allow 32-bit BFD to handle 64-bit objects
    
    We've been telling people that the riscv32-* and riscv64-* toolchains
    are exactly the same, but it turns out we were lying: the riscv32-* BFD
    doesn't handle 64-bit objects.  This fixes that difference, so the ports
    are actually the same.
    
    bfd/ChangeLog
    
    2017-05-01  Palmer Dabbelt  <palmer@dabbelt.com>
    
            * config.bfd (riscv32-*): Enable rv64.