[ELF][RISCV] Support RISC-V in getBitcodeMachineKind

Add Triple::riscv64 and Triple::riscv32 to getBitcodeMachineKind for get right
e_machine during LTO.

Reviewed By: ruiu, MaskRay

Differential Revision: https://reviews.llvm.org/D52165

llvm-svn: 364996