liblink: the zero Prog is now valid and ready for use

liblink: the zero Prog is now valid and ready for use

Use AXXX instead of AGOK (neither is a valid instruction but AXXX is zero)
for the initial setting of Prog.as, and now there are no non-zero default
field settings.

Remove the arch-specific zprog/zprg in favor of a single global zprog.
Remove the arch-specific prg constructor in favor of emallocz(sizeof(Prog)).

Change-Id: Ia73078726768333d7cdba296f548170c1bea9498
Reviewed-on: https://go-review.googlesource.com/3575
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Austin Clements <austin@google.com>