Add linux-4.15 VDSO hash for RISC-V

The RISC-V Linux port defines VDSO symbols

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

       * sysdeps/unix/sysv/linux/dl-vdso.h (VDSO_NAME_LINUX_4_15): New
       define.
       (VDSO_HASH_LINUX_4_15): Likewise.