cmd/compile: improve output when TestAssembly build fails

Change-Id: Ibee84399d81463d3e7d5319626bb0d6b60b86bd9
Reviewed-on: https://go-review.googlesource.com/40861
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/asm_test.go

@@ -152,8 +152,7 @@ func (ats *asmTests) runGo(t *testing.T, args ...string) string {
 	cmd.Stderr = &stderr
 
 	if err := cmd.Run(); err != nil {
-		fmt.Printf(stdout.String())
-		t.Fatalf("error running cmd: %v", err)
+		t.Fatalf("error running cmd: %v\nstdout:\n%sstderr:\n%s\n", err, stdout.String(), stderr.String())
 	}
 
 	if s := stderr.String(); s != "" {