Call the new llvm_gcov_init function to register the environment.

Use the new `llvm_gcov_init' function to register the writeout and flush
functions. The initialization function will also call `atexit' for some cleanups
and final writout calls. But it does this only once. This is better than
checking for the `main' function, because in a library that function may not
exist.
<rdar://problem/12439551>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177579 91177308-0d34-0410-b5e6-96231b3b80d8