Add options for building/testing with coverage.

    acov --clean
    mm -B NATIVE_COVERAGE=true ART_COVERAGE=true test-art-host
    acov --host

-B is needed because you need to be sure you rebuild *all* of ART with
coverage.

Change-Id: Ib94ef610bd1b44dc45624877710ed733051b7a50
diff --git a/runtime/Android.mk b/runtime/Android.mk
index a53616a..30cf580 100644
--- a/runtime/Android.mk
+++ b/runtime/Android.mk
@@ -481,6 +481,8 @@
     LOCAL_MODULE_TARGET_ARCH := $$(ART_TARGET_SUPPORTED_ARCH)
   endif
 
+  LOCAL_NATIVE_COVERAGE := $(ART_COVERAGE)
+
   ifeq ($$(art_target_or_host),target)
     ifneq ($$(art_ndebug_or_debug),debug)
       # Leave the symbols in the shared library so that stack unwinders can