cmd/internal/obj: make arm64 use RegTo2 instead of a full fledged Addr To2

It shrinks Prog type from 448 bytes down to 376 bytes on amd64.

It also makes sense, because I don't know of any modern architecture
that have instructions which can write to two destinations, none of
which is a register (even x86 doesn't have such instructions).

Change-Id: I3061f1c9ac93d79ee2b92ecb9049641d0e0f6300
Reviewed-on: https://go-review.googlesource.com/10330
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/link.go

@@ -206,7 +206,6 @@ type Prog struct {
 	From     Addr
 	From3    Addr
 	To       Addr
-	To2      Addr
 	Opt      interface{}
 	Forwd    *Prog
 	Pcond    *Prog
@@ -217,6 +216,7 @@ type Prog struct {
 	Spadj    int32
 	As       int16
 	Reg      int16
+	RegTo2   int16 // 2nd register output operand
 	Mark     uint16
 	Optab    uint16
 	Scond    uint8