Don't emit the extra checksum into the .gcda file if the user hasn't asked for 
it. Fortunately, versions of gcov that predate the extra checksum also ignore 
any extra data, so this isn't a problem. This matches the API change made in
r176745.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@176746 91177308-0d34-0410-b5e6-96231b3b80d8