Preparation for transition to libc++.

Move the dependency on libc++ to its own makefile so that we can switch in a
single place between libc++ and stlport.

Change-Id: Ie61e7d054dcd049e36d5e7298c27d8a4abe6edf7
diff --git a/compiler/Android.mk b/compiler/Android.mk
index b17cd52..6d656e6 100644
--- a/compiler/Android.mk
+++ b/compiler/Android.mk
@@ -158,11 +158,10 @@
   art_ndebug_or_debug := $(2)
 
   include $(CLEAR_VARS)
-  ifeq ($$(art_target_or_host),target)
-    include external/stlport/libstlport.mk
-  else
+  ifeq ($$(art_target_or_host),host)
     LOCAL_IS_HOST_MODULE := true
   endif
+  include art/build/Android.libcxx.mk
   LOCAL_CPP_EXTENSION := $(ART_CPP_EXTENSION)
   ifeq ($$(art_ndebug_or_debug),ndebug)
     LOCAL_MODULE := libart-compiler