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/compiler/Android.mk b/compiler/Android.mk
index 83ab730..55427e8 100644
--- a/compiler/Android.mk
+++ b/compiler/Android.mk
@@ -241,6 +241,9 @@
   else
     LOCAL_SHARED_LIBRARIES += libvixl
   endif
+
+  LOCAL_NATIVE_COVERAGE := $(ART_COVERAGE)
+
   ifeq ($$(art_target_or_host),target)
     # For atrace.
     LOCAL_SHARED_LIBRARIES += libcutils