cmd/objdump: fix disassembly suffixes

MOVB $1, (AX) was being disassembled as MOVL $1, (AX).

Use the memory size to override the standard size.
Fix the tests.

Fixes #15922

Change-Id: If92fe74c33a21e5427c8c5cc97dd15e087edb860
Reviewed-on: https://go-review.googlesource.com/23608Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vendor/golang.org/x/arch/x86/x86asm/plan9x.go

@@ -49,7 +49,11 @@ func GoSyntax(inst Inst, pc uint64, symname func(uint64) (string, uint64)) strin
 
 	op := inst.Op.String()
 	if plan9Suffix[inst.Op] {
-		switch inst.DataSize {
+		s := inst.DataSize
+		if inst.MemBytes != 0 {
+			s = inst.MemBytes * 8
+		}
+		switch s {
 		case 8:
 			op += "B"
 		case 16: