Add ctx.Windows()

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

Test: builds
Change-Id: I7c24c39bcf54a51ad152b21c0021e40a0c07f189
diff --git a/cc/linker.go b/cc/linker.go
index 07d9aa8..4d21d99 100644
--- a/cc/linker.go
+++ b/cc/linker.go
@@ -156,7 +156,7 @@
 		}
 	}
 
-	if ctx.Os() == android.Windows {
+	if ctx.Windows() {
 		deps.LateStaticLibs = append(deps.LateStaticLibs, "libwinpthread")
 	}