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/core/clear_vars.mk b/core/clear_vars.mk
index fb775c9..e2f1444 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -182,6 +182,7 @@
 LOCAL_SANITIZE_RECOVER:=
 LOCAL_DBUS_PROXY_PREFIX:=
 LOCAL_INIT_RC:=
+LOCAL_MODULE_HOST_OS:=
 
 # arch specific variables
 LOCAL_SRC_FILES_$(TARGET_ARCH):=