It seems like this code was broken before I messed with it. The
FFI::Config constant does not exist, so this was not actually
defining the FFI-based File.symlink. As a result, my change to
load that logic lazily caused a stack overflow.

The changes here fix this:

* Always define the Windows-specific String function present here.
* Only check for the FFI constant to know if FFI is available.
* If FFI is not available *or* it cannot bind the necessary
  functions, instead define File.symlink to raise NotImplemented.

See jruby#6751.

In jruby#4349 we saw that this logic does not load properly when native
function calls have been disabled, due to FFI partially loading
and defining an FFI module that was not fully functional. This
is why the previous code here checked for the existence of
FFI::Config rather than just FFI.

However this is still error-prone, so I am restructuring this
logic to do all FFI juggling within the same rescue block as the
require of 'ffi'. If that require raises LoadError or if the
required functions are not present, it immediately falls back on
defining the NotImplemented version of File.symlink.

The same change has been done to the Solaris File#flock logic
below.