gdb: add GDB side target_ops::fileio_stat implementation

This commit adds the GDB side of target_ops::fileio_stat.  There's an
implementation for inf_child_target, which just calls 'lstat', and
there's an implementation for remote_target, which sends a new
vFile:stat packet.

The new packet is documented.

There's still no users of target_fileio_stat as I have not yet added
support for vFile::stat to gdbserver.  If these packets are currently
sent to gdbserver then they will be reported as not supported and the
ENOSYS error code will be returned.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>