mn10300: clarify a diagnostic

[DJ Delorie]

> Hmm? We're returning bfd_reloc_dangerous for gotoff relocations
> which are, admittedly link time, but they're off the got so I don't
> know that the message is quite correct. Maybe "warning: local
> relocation against non-local symbol seen when linking a shared
> object"?

I was thinking of how you generate this error - the only way I've been
able to generate it is to link a non-"-fpic" object into a shared
library.  Maybe we could split the error case up so we can properly
inform the user?  But first we'd need to figure out how to generate
the other cases, I couldn't get any of the 8, 16, or 24 bit
relocations out of gcc.

Basically, I want to tell the user how to get rid of the error.