[RISCV] Support -mcpu/mtune=native

We may need hosted Clang/LLVM to compile and `getHostCpuName`
can be used for native detection.

Tests are added in riscv-cpus.c just like what AArch64/PPC
have done.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D136930