RFC: RISC-V: Add new -mtune syntax

[Sam James]

Kito Cheng via Gcc <gcc@gcc.gnu.org> writes:

> Hi:
>
> My colleague Min is proposing a new syntax for -mtune[1] which makes
> the mtune able to describe few more info and is configurable like
> "-mtune=rocket:short-forward-branch-i-mul".

The syntax is a bit like x86's -mtune-ctrl, but for -mtune-ctrl, we say
it is only for GCC developers. I don't know if choosing a similar name
is appropriate then (was wondering if riscv should use -mtune-ctrl).