Add RISC-V dynamic relocations to elf.h

These relocations can appear in shared objects on RISC-V ELF systems.

2018-01-06  Palmer Dabbelt  <palmer@sifive.com>

       * elf/elf.h (R_RISCV_NONE): New define.
       (R_RISCV_32): Likewise.
       (R_RISCV_64): Likewise.
       (R_RISCV_RELATIVE): Likewise.
       (R_RISCV_COPY): Likewise.
       (R_RISCV_JUMP_SLOT): Likewise.
       (R_RISCV_TLS_DTPMOD32): Likewise.
       (R_RISCV_TLS_DTPMOD64): Likewise.
       (R_RISCV_TLS_DTPREL32): Likewise.
       (R_RISCV_TLS_DTPREL64): Likewise.
       (R_RISCV_TLS_TPREL32): Likewise.
       (R_RISCV_TLS_TPREL64): Likewise.