Add some GCOV functions that register all of the __llvm_gcov_flush() functions.

The __llvm_gcov_flush() functions only work for the local compile unit. However,
when __gcov_flush() is called, the user expects all of the counters to be
flushed, not just the ones in the current compile unit.

This adds some library functions that register the flush functions. It also
defined __gcov_flush() so that loops through that list and calls the functions.

PR15191 & <rdar://problem/13167507>

llvm-svn: 177337