[RISCV] Recognize new relocation types

This patch adds all RISC-V relocation types, as of binutils 2.29. Note that 
R_RISCV32_PCREL is not currently documented in the RISC-V ELF PSABI.

Differential Revision: https://reviews.llvm.org/D36455

Patch by Chih-Mao Chen (@PkmX)

llvm-svn: 310914