xstormy2 pcrel reloc fix

[DJ Delorie]

> If it used to work once, and it stopped, you should fix the thing
> that made it stop working, rather than add another mechanism.

I just built an xstormy16-elf from a year ago, just after it was
initially added.  It was broken then.

> Either way, though, a testcase would be a good idea.

reloc-2 already tests this.  It would need to be adjusted to
compensate for this change.