Create a coverage initialization function.

This function replaces the call of `atexit' from being generated in the compile
units. Basically, it registers the "writeout" and "flush" functions (if
present). It will generate calls to the `atexit' function for cleanups and final
writeout functions, but only once. This is better than checking for `main',
because a library may not have a `main' function in it.
<rdar://problem/12439551>


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177578 91177308-0d34-0410-b5e6-96231b3b80d8