Add _(darwin|linux|windows)

So that we can support building both linux and windows binaries at the
same time on a linux host. This replaces the ifeq($(HOST_OS),...) checks
in Android.mk files.

Bug: 23566667

Change-Id: I693e11984e36d55bb6f09fa0d49bc485463e16fb
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c42a8e0..63ae2a2 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -255,6 +255,18 @@
 LOCAL_CLANG_$(HOST_2ND_ARCH):=
 endif
 
+LOCAL_SRC_FILES_$(HOST_OS):=
+LOCAL_STATIC_LIBRARIES_$(HOST_OS):=
+LOCAL_SHARED_LIBRARIES_$(HOST_OS):=
+LOCAL_CFLAGS_$(HOST_OS):=
+LOCAL_CPPFLAGS_$(HOST_OS):=
+LOCAL_LDFLAGS_$(HOST_OS):=
+LOCAL_LDLIBS_$(HOST_OS):=
+LOCAL_ASFLAGS_$(HOST_OS):=
+LOCAL_C_INCLUDES_$(HOST_OS):=
+LOCAL_GENERATED_SOURCES_$(HOST_OS):=
+LOCAL_REQUIRED_MODULES_$(HOST_OS):=
+
 LOCAL_SRC_FILES_32:=
 LOCAL_SRC_FILES_64:=
 LOCAL_SHARED_LIBRARIES_32:=