os: skip TestHardLink on Android.

From Android release M (Marshmallow), hard linking files is blocked
and an attempt to call link() on a file will return EACCES.
- https://code.google.com/p/android-developer-preview/issues/detail?id=3150

Change-Id: Ifdadaa31e3d5ee330553f45db6c001897dc955be
Reviewed-on: https://go-review.googlesource.com/17339Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -583,6 +583,12 @@ func TestHardLink(t *testing.T) {
 	if runtime.GOOS == "plan9" {
 		t.Skip("skipping on plan9, hardlinks not supported")
 	}
+	// From Android release M (Marshmallow), hard linking files is blocked
+	// and an attempt to call link() on a file will return EACCES.
+	// - https://code.google.com/p/android-developer-preview/issues/detail?id=3150
+	if runtime.GOOS == "android" {
+		t.Skip("skipping on android, hardlinks not supported")
+	}
 	defer chtmpdir(t)()
 	from, to := "hardlinktestfrom", "hardlinktestto"
 	Remove(from) // Just in case.