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/base_rules.mk b/core/base_rules.mk
index b936859..3fedabc 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -645,9 +645,13 @@
 ALL_MODULES.$(my_register_name).PICKUP_FILES := \
     $(ALL_MODULES.$(my_register_name).PICKUP_FILES) $(LOCAL_PICKUP_FILES)
 endif
+my_required_modules := $(LOCAL_REQUIRED_MODULES) \
+    $(LOCAL_REQUIRED_MODULES_$(TARGET_$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH))
+ifdef LOCAL_IS_HOST_MODULE
+my_required_modules += $(LOCAL_REQUIRED_MODULES_$($(my_prefix)OS))
+endif
 ALL_MODULES.$(my_register_name).REQUIRED := \
-    $(strip $(ALL_MODULES.$(my_register_name).REQUIRED) $(LOCAL_REQUIRED_MODULES) \
-      $(LOCAL_REQUIRED_MODULES_$(TARGET_$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)))
+    $(strip $(ALL_MODULES.$(my_register_name).REQUIRED) $(my_required_modules))
 ALL_MODULES.$(my_register_name).EVENT_LOG_TAGS := \
     $(ALL_MODULES.$(my_register_name).EVENT_LOG_TAGS) $(event_log_tags)
 ALL_MODULES.$(my_register_name).INTERMEDIATE_SOURCE_DIR := \