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.