Register the GCOV writeout functions so that they're emitted serially.

We don't want to write out >1000 files at the same time. That could make things
prohibitively expensive. Instead, register the "writeout" function so that it's
emitted serially.
<rdar://problem/12439551>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177437 91177308-0d34-0410-b5e6-96231b3b80d8