[rfa] Always define all of TARGET_SIGNAL_*

[Andrew Cagney]

> I've been meaning to fix this since I noticed it last summer.  These numbers
> are part of the remote protocol.  While I think the last ones are never sent
> over the wire, they could be (with the exception of TARGET_SIGNAL_LAST which
> is -not- part of the protocol, as I understand it).  Having them jump around
> is bad.

Wasn't this enum going to be moved to signals.h?

Andrew