cmd/gc: remove C subclassing trick from popt.c

cmd/gc: remove C subclassing trick from popt.c

It does not convert to Go well.

Being able to do this just once, instead of 4 times, was the primary
motivation for all the recent refactoring (not that it wasn't overdue).

Still bit-for-bit identical.

Change-Id: Ia01f17948441bf64fa78ec4226f0bb40af0bbaab
Reviewed-on: https://go-review.googlesource.com/3962
Reviewed-by: Austin Clements <austin@google.com>