Add LOCAL_MODULE_HOST_OS

Instead of wrapping a host module definition in 'ifeq($(HOST_OS),...)'
in the Android.mk files, define which hosts are supported using
LOCAL_MODULE_HOST_OS.

A blank LOCAL_MODULE_HOST_OS means that linux and darwin are supported.
A non-empty LOCAL_MODULE_HOST_OS lists the supported HOST_OSs.

Change-Id: I1e342d1908cfa00aef2c39c145b4f5f81c373bc6
diff --git a/tools/zipalign/Android.mk b/tools/zipalign/Android.mk
index cba6db3..8c0240a 100644
--- a/tools/zipalign/Android.mk
+++ b/tools/zipalign/Android.mk
@@ -33,5 +33,6 @@
 endif # BUILD_HOST_static
 
 LOCAL_MODULE := zipalign
+LOCAL_MODULE_HOST_OS := darwin linux windows
 
 include $(BUILD_HOST_EXECUTABLE)