[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

llvm/tools/llvm-objdump/llvm-objdump.cpp

@@ -704,6 +704,7 @@ static std::error_code getRelocationValueString(const ELFObjectFile<ELFT> *Obj,
   case ELF::EM_HEXAGON:
   case ELF::EM_MIPS:
   case ELF::EM_BPF:
+  case ELF::EM_RISCV:
     res = Target;
     break;
   case ELF::EM_WEBASSEMBLY: