gdb: add target_fileio_stat, but no implementations yet

In a later commit I want target_fileio_stat, that is a call that
operates on a filename rather than an open file descriptor as
target_fileio_fstat does.

This commit adds the initial framework for target_fileio_stat, I've
added the top level target function and the virtual target_ops methods
in the target_ops base class.

At this point no actual targets override target_ops::fileio_stat, so
any attempts to call this function will return ENOSYS error code.