cmd/internal/ld: replace Diag;Errorexit with Exitf

cmd/internal/ld: replace Diag;Errorexit with Exitf

I have left the Diag calls in place where I believe Ctxt.Cursym != nil
which means this CL is not the improvement I had hoped for. However
it is now safe to call Exitf whereever you are in the linker, which
makes it easier to reason about some code.

Change-Id: I8261e761ca9719f7d216e2747314adfe464e3337
Reviewed-on: https://go-review.googlesource.com/8668
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>