[RISCV] Add RISC-V ELF defines

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285708 91177308-0d34-0410-b5e6-96231b3b80d8