[RISCV] Add the RISCV target and compiler driver

As RV64 codegen has not yet been upstreamed into LLVM, we focus on RV32 driver 
support (RV64 to follow).

Differential Revision: https://reviews.llvm.org/D39963


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@322276 91177308-0d34-0410-b5e6-96231b3b80d8