cmd/internal/obj: unify creation of numeric literal syms

cmd/internal/obj: unify creation of numeric literal syms

This is a straightforward refactoring,
to reduce the scope of upcoming changes.

The symbol size and AttrLocal=true was not
set universally, but it appears not to matter,
since toolstash -cmp is happy.

Passes toolstash-check -all.

Change-Id: I7f8392f939592d3a1bc6f61dec992f5661f42fca
Reviewed-on: https://go-review.googlesource.com/39791
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>