cmd/internal/obj: make Dconv a portable top-level function

Each architecture had its own Dconv (operand printer) but the syntax is
close to uniform and the code overlap was considerable. Consolidate these
into a single top-level function. A similar but smaller unification is done
for Mconv ("Name" formatter) as well.

The signature is changed. The flag was unused so drop it. Add a
function argument, Rconv, that must be supplied by the caller.
TODO: A future change will unify Rconv as well and this argument
will go away.

Some formats changed, because of the automatic consistency
created by unification. For instance, 0(R1) always prints as (R1)
now, and foo+0(SB) is just foo(SB). Before, some made these
simplifications and some didn't; now they all do.

Update the asm tests that depend on the format.

Change-Id: I6e3310bc19814c0c784ff0b960a154521acd9532
Reviewed-on: https://go-review.googlesource.com/5920
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/x86/obj6.go

@@ -1111,7 +1111,6 @@ loop:
 }
 
 var Linkamd64 = obj.LinkArch{
-	Dconv:      Dconv,
 	Rconv:      Rconv,
 	ByteOrder:  binary.LittleEndian,
 	Pconv:      Pconv,
@@ -1128,7 +1127,6 @@ var Linkamd64 = obj.LinkArch{
 }
 
 var Linkamd64p32 = obj.LinkArch{
-	Dconv:      Dconv,
 	Rconv:      Rconv,
 	ByteOrder:  binary.LittleEndian,
 	Pconv:      Pconv,