x86/APX: correct libbfd's EVEX Rn -> Bn transformations

x86/APX: correct libbfd's EVEX Rn -> Bn transformations

In the recent GOTPCREL addition I screwed up, in clearing the Rn bits
afterwards rather than setting them. While that ought to be benign (for
the bits being ignored in situations like this), we still want to leave
"canonical" encodings.

The pre-existing GOTTPOFF conversion wasn't doing quite correctly
either: We cannot assume the incoming Bn bits to be in a particular
state, as for the addressing form in question they're ignored as well.

To address both, introduce a helper function. This is then also an
overall reduction of (source) code size (and use of "magic" numbers).