[RISCV] Add RISC-V ELF defines

Add the necessary definitions for RISC-V ELF files, including relocs. Also 
make necessary trivial change to ELFYaml, llvm-objdump, and llvm-readobj in 
order to work with RISC-V ELFs.

Differential Revision: https://reviews.llvm.org/D23557

llvm-svn: 285708