Add HOST_CROSS_OS

Instead of using recursive make to change the HOST_OS when building the
windows SDK under linux, add the concept of cross-building to another
host os.

Bug: 23566667
Change-Id: I6dc525b601b6251d458d197c30bf4660d7485502
diff --git a/core/clang/config.mk b/core/clang/config.mk
index 910c1a4..03d7730 100644
--- a/core/clang/config.mk
+++ b/core/clang/config.mk
@@ -84,6 +84,12 @@
 CLANG_CONFIG_HOST_EXTRA_CPPFLAGS :=
 CLANG_CONFIG_HOST_EXTRA_LDFLAGS :=
 
+# Clang flags for all host cross rules
+CLANG_CONFIG_HOST_CROSS_EXTRA_ASFLAGS :=
+CLANG_CONFIG_HOST_CROSS_EXTRA_CFLAGS :=
+CLANG_CONFIG_HOST_CROSS_EXTRA_CPPFLAGS :=
+CLANG_CONFIG_HOST_CROSS_EXTRA_LDFLAGS :=
+
 # Clang flags for all target rules
 CLANG_CONFIG_TARGET_EXTRA_ASFLAGS :=
 CLANG_CONFIG_TARGET_EXTRA_CFLAGS := -nostdlibinc
@@ -126,6 +132,10 @@
 include $(BUILD_SYSTEM)/clang/HOST_$(HOST_2ND_ARCH).mk
 endif
 
+ifdef HOST_CROSS_OS
+include $(BUILD_SYSTEM)/clang/HOST_CROSS_$(HOST_CROSS_OS).mk
+endif
+
 # TARGET config
 clang_2nd_arch_prefix :=
 include $(BUILD_SYSTEM)/clang/TARGET_$(TARGET_ARCH).mk