cmd/link: skip TestFieldOverlap on Plan 9

TestSizes has been added in CL 50170. This test is
failing on Plan 9 because executables don't have
a DWARF symbol table.

Fixes #21480.

Change-Id: I51079abdc18ad944617bdbcfe2dad970a0cea0f2
Reviewed-on: https://go-review.googlesource.com/56210
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -241,6 +241,10 @@ func main() {
 }
 
 func TestFieldOverlap(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	// This test grew out of issue 21094, where specific sudog<T> DWARF types
 	// had elem fields set to values instead of pointers.
 	const prog = `