Add ctx.Windows()

Replace ctx.Os() == android.Windows with ctx.Windows()

Test: builds
Change-Id: I7c24c39bcf54a51ad152b21c0021e40a0c07f189
diff --git a/cc/stl.go b/cc/stl.go
index 43776f7..9e67145 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -60,7 +60,7 @@
 				ctx.ModuleErrorf("stl: %q is not a supported STL with sdk_version set", s)
 				return ""
 			}
-		} else if ctx.Os() == android.Windows {
+		} else if ctx.Windows() {
 			switch s {
 			case "libc++", "libc++_static", "libstdc++", "":
 				// libc++ is not supported on mingw