cmd/asm: reject foo(SB)(AX) instead of silently treating as foo(SB)

Add test for assembly errors, to verify fix.
Make sure invalid instruction errors are printed just once
(was printing them once per span iteration, so typically twice).

Fixes #13282.

Change-Id: Id5f66f80a80b3bc4832e00084b0a91f1afec7f8f
Reviewed-on: https://go-review.googlesource.com/18858
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/internal/obj/link.go

@@ -604,12 +604,13 @@ type Link struct {
 	Autosize           int32
 	Armsize            int32
 	Pc                 int64
-	Diag               func(string, ...interface{})
+	DiagFunc           func(string, ...interface{})
 	Mode               int
 	Cursym             *LSym
 	Version            int
 	Textp              *LSym
 	Etextp             *LSym
+	Errors             int
 
 	// state for writing objects
 	Text  *LSym
@@ -618,6 +619,11 @@ type Link struct {
 	Edata *LSym
 }
 
+func (ctxt *Link) Diag(format string, args ...interface{}) {
+	ctxt.Errors++
+	ctxt.DiagFunc(format, args...)
+}
+
 // The smallest possible offset from the hardware stack pointer to a local
 // variable on the stack. Architectures that use a link register save its value
 // on the stack in the function prologue and so always have a pointer between