Don't use "_gp" on RISC-V, use "_global_pointer$" instead

"_gp" could conflict with ABI-complient code.  While it's probably OK
because MIPS uses this name, we figured it'd be good to clean this up
before a release with RISC-V in it.

ld/ChangeLog:

2017-02-13  Palmer Dabbelt  <palmer@dabbelt.com>

        * emulparams/elf32lriscv-defs.sh (SDATA_START_SYMBOLS): Change
        _gp to __global_pointer$.

bfd/ChangeLog:

2017-02-13  Palmer Dabbelt  <palmer@dabbelt.com>

        * elfnn-riscv.c (riscv_global_pointer_value): Change _gp to
        __global_pointer$.