[RISCV][Docs] Use double underscore for external links in RISCVUsage.rst.

Using a single underscore creates a reference target. If the target
name has the same name as another link, we get a "Duplicate target name"
warning. This is currently happening for Ssqosid.

Using __ prevents this. I've converted all links so no one trips over
this in the future.

One link was missing any underscores so wasn't a link at all in the
generated html.