RISC-V: Resurrect GP-relative disassembly hints
    
    We missed a "_gp" when changing the GP symbol.  To make sure that
    doesn't happen again, we now use the same definition everywhere (thanks,
    Nick).
    
    include/ChangeLog:
    
    2017-04-03  Palmer Dabbelt  <palmer@dabbelt.com>
    
            * elf/riscv.h (RISCV_GP_SYMBOL): New define.
    
    bfd/ChangeLog:
    
    2017-04-03  Palmer Dabbelt  <palmer@dabbelt.com>
    
            * elfnn-riscv.c (GP_NAME): Delete.
            (riscv_global_pointer_value): Change GP_NAME to RISCV_GP_SYMBOL.
            (_bfd_riscv_relax_lui): Likewise.
    
    opcodes/ChangeLog:
    
    2017-04-03  Palmer Dabbelt  <palmer@dabbelt.com>
    
            * riscv-dis.c (riscv_disassemble_insn): Change "_gp" to
            RISCV_GP_SYMBOL.