llvm/cmake/config.guess: add support for riscv32 and riscv64

Summary: LLVM configuration fails with 'unable to guess system type' on riscv64.
Add support for detecting riscv32 and riscv64 systems.

Patch by Gokturk Yuksek (gokturk)
Reviewers: erichkeane, rengolin, mgorny, aaron.ballman, beanz, luismarques
Reviewed By: luismarques
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D68899