[sanitizer] Support sandboxing in sanitizer coverage.

Summary:
Sandboxed code may now pass additional arguments to
__sanitizer_sandbox_on_notify() to force all coverage data to be dumped to a
single file (the default is one file per module). The user may supply a file or
socket to write to. The latter option can be used to broker out the file writing
functionality. If -1  is passed, we pre-open a file.