internal/xcoff: fix wrong bit masking comparisons

I do not know much about xcoff, but this was probably the intended
behavior. (The comparison is tautologically false, as is.)

Also note: does any other code even depend on the changed code existing?
Maybe it should just be removed, as I did not find any uses of fields
that are written to if the branch condition tests true.

Change-Id: I1f23d33764df40e87f3e64460d63f6efc51a2a78
GitHub-Last-Rev: 268909130fd7fb3993fcf7004143ec48dbfe5e2a
GitHub-Pull-Request: golang/go#37733
Reviewed-on: https://go-review.googlesource.com/c/go/+/222478


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Clément Chigot <clement.chigot%atos.net@gtempaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>