[FEATURE REQUEST] ifence

[rkabrick]

Rev Feature Request
A clear and concise description of what the problem is.

ifence

Version of SST
Describe the version of SST where you would like this to run

13.x.x

Additional context
ifence

[leekillough]

In #240 I have separated Zifencei as a separate extension.

AFAIK, fence.i was already somewhat implemented, but it was not separated as an extension, as it is in my PR, because it was originally part of the core RV{32,64}I.

What precisely does this issue request?