This commit adds a specialized version of DiscardOutput for fuchsia,
removing an usage of /dev/null.
In fuchsia, accessing /dev/null is not supported, and there's nothing
similar to a file that discards everything that is written to it. The
way of doing something similar in fuchsia is by using fdio_null_create
and binding that to a file descriptor with fdio_bind_to_fd.
This change should fix one of the issues with the -close_fd_mask flag
in libfuzzer, in which closing stdout was not working due to
fopen("/dev/null", "w") returning NULL.