SuperH literal pools in gas

[Joseph Matthews]

Hi Oleg,

I'm not familiar with how gas works internally, so I'm not confident in my
ability to produce such a patch, especially as SH support seems to be quite
brittle in its current state. I'll consider giving it a go though.

On the other hand, it turns out that sp _is_ an alias to r15, however it
currently goes undocumented in c-sh.texi. That I can amend.

Joseph