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/build/Android.gtest.mk b/build/Android.gtest.mk
index a5cfcba..b4eb0c0 100644
--- a/build/Android.gtest.mk
+++ b/build/Android.gtest.mk
@@ -275,7 +275,7 @@
 	$(hide) adb shell rm $(ART_TARGET_TEST_DIR)/$(TARGET_$(2)ARCH)/$$@-$$$$PPID
 	$(hide) adb shell chmod 755 $(ART_TARGET_NATIVETEST_DIR)/$(TARGET_$(2)ARCH)/$(1)
 	$(hide) $$(call ART_TEST_SKIP,$$@) && \
-	  (adb shell "LD_LIBRARY_PATH=$(3) ANDROID_ROOT=$(ART_GTEST_TARGET_ANDROID_ROOT) \
+	  (adb shell "$(GCOV_ENV) LD_LIBRARY_PATH=$(3) ANDROID_ROOT=$(ART_GTEST_TARGET_ANDROID_ROOT) \
 	    $(ART_TARGET_NATIVETEST_DIR)/$(TARGET_$(2)ARCH)/$(1) && touch $(ART_TARGET_TEST_DIR)/$(TARGET_$(2)ARCH)/$$@-$$$$PPID" \
 	  && (adb pull $(ART_TARGET_TEST_DIR)/$(TARGET_$(2)ARCH)/$$@-$$$$PPID /tmp/ \
 	      && $$(call ART_TEST_PASSED,$$@)) \