[Driver][RISCV] Adjust the priority between -mcpu, -mtune and -march

[Driver][RISCV] Adjust the priority between -mcpu, -mtune and -march

RISC-V supports `-march`, `-mtune`, and `-mcpu`: `-march` provides the
architecture extension information, `-mtune` provide the pipeline model, and
`-mcpu` provides both.

What's the priority among those options for now(w/o this patch)?

Pipeline model:
- Take from `-mtune` if present.
- Take from `-mcpu` if present
- Use the default pipeline model: `generic-rv32` or `generic-rv64`
Architecture extension has quite complicated behavior now:
- Take union from `-march` and `-mcpu` if both are present.
- Take from `-march` if present.
- Take from `-mcpu` if present.
- Implied from `-mabi` if present.
- Use the default architecture depending on the target triple

We treat `-mcpu`/`-mtune` and `-mcpu`/`-march` differently, and it's
kind of counterintuitive: -march is explicitly specified but ignored.

This patch adjusts the priority between `-mcpu`/`-march`, letting it use
architecture extension information from `-march` if it's present.

So the priority of architecture extension information becomes:
- Take from `-march` if present.
- Take from `-mcpu` if present.
- Implied from `-mabi` if present.
- Use the default architecture depending on the target triple

And this also match what we implement in RISC-V GCC too.

Reviewed By: craig.topper, MaskRay

Differential Revision: https://reviews.llvm.org/D140693