[RISCV] Add support for -mcpu option.
- gcc uses -march and -mtune flag to chose arch and
pipeline model, but clang does not have -mtune flag,
we uses -mcpu to chose both infos.
- Add SiFive e31 and u54 cpu which have default march
and pipeline model.
- Specific -mcpu with rocket-rv[32|64] would select
pipeline model only, and use the driver's arch choosing
logic to get default arch.
Reviewers: lenary, asb, evandro, HsiangKai
Reviewed By: lenary, asb, evandro
Tags: #llvm, #clang
Differential Revision: https://reviews.llvm.org/D71124