[RISCV] Add support for -mcpu option.

Summary:
1. 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.
2. Add SiFive e31 and u54 cpu which have default march
and pipeline model.
3. 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