cmd/internal/obj: delete all Pconv, replace with Prog.String

Remove the per-achitecture formatter for Prog and replace it with
a global String method. Clean up and regularize the output. Update
tests affected by the format; some tests are made correct now when
they were broken before (and known to be).

Also, related: Change the encoding of the (R1+R2) syntax on ppc64
to be equivalent to (R1)(R2*1), which means it needs no special
handling.

Delete the now unused STRINGSZ constant.

Change-Id: I7f6654d11f80065f3914a3f19353f2f12edfe310
Reviewed-on: https://go-review.googlesource.com/6931
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/link.go

@@ -234,7 +234,6 @@ type Plist struct {
 }
 
 type LinkArch struct {
-	Pconv      func(*Prog) string
 	ByteOrder  binary.ByteOrder
 	Name       string
 	Thechar    int
@@ -382,7 +381,16 @@ type Pciter struct {
 //			offset = bit mask of registers in list; R0 is low bit.
 //
 //	reg, reg
-//		TYPE_REGREG2, to be removed.
+//		Register pair for ARM.
+//		TYPE_REGREG2
+//
+//	(reg+reg)
+//		Register pair for PPC64.
+//		Encoding:
+//			type = TYPE_MEM
+//			reg = first register
+//			index = second register
+//			scale = 1
 //
 
 const (