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/binary.mk b/core/binary.mk
index 01c8858..7b629d8 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -153,6 +153,19 @@
 endif
 endif
 
+ifdef LOCAL_IS_HOST_MODULE
+my_src_files += $(LOCAL_SRC_FILES_$($(my_prefix)OS))
+my_static_libraries += $(LOCAL_STATIC_LIBRARIES_$($(my_prefix)OS))
+my_shared_libraries += $(LOCAL_SHARED_LIBRARIES_$($(my_prefix)OS))
+my_cflags += $(LOCAL_CFLAGS_$($(my_prefix)OS))
+my_cppflags += $(LOCAL_CPPFLAGS_$($(my_prefix)OS))
+my_ldflags += $(LOCAL_LDFLAGS_$($(my_prefix)OS))
+my_ldlibs += $(LOCAL_LDLIBS_$($(my_prefix)OS))
+my_asflags += $(LOCAL_ASFLAGS_$($(my_prefix)OS))
+my_c_includes += $(LOCAL_C_INCLUDES_$($(my_prefix)OS))
+my_generated_sources += $(LOCAL_GENERATED_SOURCES_$($(my_prefix)OS))
+endif
+
 my_src_files += $(LOCAL_SRC_FILES_$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)) $(LOCAL_SRC_FILES_$(my_32_64_bit_suffix))
 my_shared_libraries += $(LOCAL_SHARED_LIBRARIES_$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)) $(LOCAL_SHARED_LIBRARIES_$(my_32_64_bit_suffix))
 my_cflags += $(LOCAL_CFLAGS_$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)) $(LOCAL_CFLAGS_$(my_32_64_bit_suffix))