gas: constify md_{short,long}opts and md_longopts_size

gas: constify md_{short,long}opts and md_longopts_size

First of all make the declarations globally visible, such that producer
and consumer actually share them.

For the latter two simply add const (as PPC already had it,), while for
the former achieve the effect by converting to an array: There's no need
for the extra level of indirection.