`riscv`: Support more fence variants

[jsgf]

The default fence instruction with no parameters is the strongest form of barrier. But it also takes an optional set of flags to give fine-grained control over exactly what operations the fence is a barrier for, and in which direction.

I see two design choices here:

• just let the caller specify an arbitrary set of flags (somehow), or
• have a set of canned variants for the common cases (like Linux's rmb/wmb/...)

(These aren't exclusive of course.)

Thoughts?

cc @dreiss

[romancardenas]

Hi! I need to think more about this, but currently, I think that riscv should support common cases. For very specific scenarios, developers could always use inline assembly.