liblink: place TEXT/GLOBL flags in p->from3 always
Before, amd64 and 386 stored the flags in p->from.scale
and arm and ppc64 stored the flags in p->reg.
Both caused special cases in printing and in handling of the
addresses.
To avoid possible conflicts with the real meaning of p->from
and to avoid storing a non-register value in a reg field,
use from3 to hold a TYPE_CONST value giving the flags.
There is still a special case for printing, because the flags
are specified without a $, and normally a TYPE_CONST prints
with a $. But that's much less special than what came before.
This allows us to remove the textflag and settextflag methods
from LinkArch. They are no longer architecture-specific.