[RISCV] Recognise riscv32 and riscv64 in triple parsing code

This is the first in a series of 10 initial patches that incrementally add an 
MC layer for RISC-V to LLVM. See 
<http://lists.llvm.org/pipermail/llvm-dev/2016-August/103748.html> for more 
discussion.

Differential Revision: https://reviews.llvm.org/D23557

llvm-svn: 285707