Make test_for arch variant.

To be able to avoid registering dependencies on nonexistent host APEXes
in host builds.

Test: art/tools/buildbot-build.sh --host
  with http://r.android.com/q/topic:libdexfile-noext applied
Bug: 183217299
Change-Id: Iaa6411b511b6f50da01827b49852607ae825bc83

cc/cc.go

@@ -363,7 +363,7 @@ type BaseProperties struct {
 	// List of APEXes that this module has private access to for testing purpose. The module
 	// can depend on libraries that are not exported by the APEXes and use private symbols
 	// from the exported libraries.
-	Test_for []string
+	Test_for []string `android:"arch_variant"`
 }
 
 type VendorProperties struct {