liblink: define fixed A-numbers for common instructions

liblink: define fixed A-numbers for common instructions

This makes names like ANOP, ATEXT, AGLOBL, ACALL, AJMP, ARET
available for use by architecture-independent processing passes.

On arm and ppc64, the alternate names are now aliases for the
official ones (ABL for ACALL, AB or ABR for AJMP, ARETURN for ARET).

Change-Id: Id027771243795af2b3745199c645b6e1bedd7d18
Reviewed-on: https://go-review.googlesource.com/3577
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Austin Clements <austin@google.com>