[RISCV] Add ELFObjectFileBase::getRISCVFeatures let llvm-objdump could get RISCV target feature

llvm-objdump could get C feature by ELF::EF_RISCV_RVC e_flag,
so then we don't have to add -mattr=+c on the command line.

Differential Revision: https://reviews.llvm.org/D42629

llvm-svn: 324058