cmd/link: skip TestSizes on Plan 9

TestSizes has been added in CL 55551. This test is
failing on Plan 9 because executables don't have
a DWARF symbol table.

Fixes #21453.

Change-Id: I560611b49aea5417e8c5ac0cec6c7882bd9f8335
Reviewed-on: https://go-review.googlesource.com/55692
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>

src/cmd/link/internal/ld/dwarf_test.go

@@ -194,6 +194,10 @@ func findMembers(rdr *dwarf.Reader) (map[string]bool, error) {
 }
 
 func TestSizes(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	// DWARF sizes should never be -1.
 	// See issue #21097
 	const prog = `