[RISCV] Recognise riscv32 and riscv64 in triple parsing code

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@285707 91177308-0d34-0410-b5e6-96231b3b80d8