make coverage should execute sequentially

Summary:
For some reason, parallel tests are failing in Jenkins. We turned off parallel tests for all tests except `make coverage`. Let's turn it off here, too, so that we can have Jenkins test working again.

For the error log, see: https://ci-builds.fb.com/job/rocksdb_coverage/166/console

Test Plan: commit and wait for jenkins

Reviewers: meyering, sdong, anthony

Reviewed By: anthony

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D42057

Makefile

@@ -376,7 +376,7 @@ release:
 
 coverage:
 	$(MAKE) clean
-	COVERAGEFLAGS="-fprofile-arcs -ftest-coverage" LDFLAGS+="-lgcov" $(MAKE) all check
+	COVERAGEFLAGS="-fprofile-arcs -ftest-coverage" LDFLAGS+="-lgcov" $(MAKE) J=1 all check
 	cd coverage && ./coverage_test.sh
         # Delete intermediate files
 	find . -type f -regex ".*\.\(\(gcda\)\|\(gcno\)\)" -exec rm {} \;