cmd/internal/obj: move fields from obj.Link to x86.AsmBuf

These fields are used to encode a single instruction.
Add them to AsmBuf, which is also per-instruction,
and which is not global.

Updates #15756

Change-Id: I0e5ea22ffa641b07291e27de6e2ff23b6dc534bd
Reviewed-on: https://go-review.googlesource.com/38668
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/link.go

@@ -735,11 +735,6 @@ type Link struct {
 	Printp        *Prog
 	Blitrl        *Prog
 	Elitrl        *Prog
-	Rexflag       int
-	Vexflag       int
-	Rep           int
-	Repn          int
-	Lock          int
 	Instoffset    int64
 	Autosize      int32
 	Armsize       int32