[sanitizer coverage] write the pc-table at the process exit

The current code writes the pc-table at the process startup,
which may happen before the common_flags() are initialized.
Move writing to the process end.
This is consistent with how we write the counters and avoids the problem with the uninitalized flags.
Add prints if verbosity>=1.

Reviewed By: kostik

Differential Revision: https://reviews.llvm.org/D110119