[gcov] Move llvm_writeout_files from atexit to a static destructor
atexit registered functions run earlier so __attribute__((destructor))
annotated functions cannot be tracked.
Set a priority of 100 (compatible with GCC 7 onwards) to track
destructors and destructors whose priorities are greater than 100.
Reviewed By: calixte, marco-c
Differential Revision: https://reviews.llvm.org/D82253