Allow -lobjc and -framework IOKit in darwin host_ldlibs.

Bug: http://b/31321337
Test: mma of external/libusb on darwin
Change-Id: I1023a2a7ff1fdf809ac47104bca7b4171861abef

cc/config/x86_darwin_host.go

@@ -91,14 +91,19 @@ var (
 		"10.11",
 	}
 
-	darwinAvailableLibraries = append(addPrefix([]string{
+	darwinAvailableLibraries = append(
+		addPrefix([]string{
 			"c",
 			"dl",
 			"m",
 			"ncurses",
+			"objc",
 			"pthread",
 			"z",
-	}, "-l"), "-framework CoreFoundation")
+		}, "-l"),
+		"-framework CoreFoundation",
+		"-framework IOKit",
+	)
 )
 
 const (