[lsan] On Fuchsia, don't use atexit hook for leak checks

This is a re-land of https://reviews.llvm.org/D86171 with fix.

Fuchsia's system libraries are instrumented and use the lsan
allocator for internal purposes.  So leak checking needs to run
after all atexit hooks and after the system libraries' internal
exit-time hooks.  The <zircon/sanitizer.h> hook API calls the
__sanitizer_process_exit_hook function at exactly the right time.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D88248