Find the correct gcov (#6904)

Summary:
Right now in FB environment, wrong gcov is used. Fix it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6904

Test Plan: "make coverage" and watch results.

Reviewed By: riversand963

Differential Revision: D21824291

fbshipit-source-id: 666011fd86c36adafa09ebd9eb97742f94fb90bb

coverage/coverage_test.sh

@@ -12,7 +12,7 @@ fi
 ROOT=".."
 # Fetch right version of gcov
 if [ -d /mnt/gvfs/third-party -a -z "$CXX" ]; then
-  source $ROOT/build_tools/fbcode_config.sh
+  source $ROOT/build_tools/fbcode_config_platform007.sh
   GCOV=$GCC_BASE/bin/gcov
 else
   GCOV=$(which gcov)