Consistently prepend arch-specific headers

Add variant_prepend to Export_include_dirs, Export_system_include_dirs
and Generated_headers to match Local_include_dirs so that
architecture-specific headers are found before common headers.

Test: m checkbuild
Test: TestIncludeDirectoryOrdering
Change-Id: I2bf9d4373fd256e500c1f8d6c84cf593e8fdd170
diff --git a/cc/library.go b/cc/library.go
index 93bd56c..196d575 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -183,11 +183,11 @@
 	// be added to the include path (using -I) for this module and any module that links
 	// against this module.  Directories listed in export_include_dirs do not need to be
 	// listed in local_include_dirs.
-	Export_include_dirs []string `android:"arch_variant"`
+	Export_include_dirs []string `android:"arch_variant,variant_prepend"`
 
 	// list of directories that will be added to the system include path
 	// using -isystem for this module and any module that links against this module.
-	Export_system_include_dirs []string `android:"arch_variant"`
+	Export_system_include_dirs []string `android:"arch_variant,variant_prepend"`
 
 	Target struct {
 		Vendor, Product struct {