swank-loader: use defaults when computing src dir

(Essential on Windows, otherwise the drive is not picked up.)
Can't use Gray streams for ABCL

* swank/abcl.lisp (make-output-stream, make-input-stream): Use
ext:make-slime-output-stream because ABCL's Gray streams don't
work well enough.
* swank-loader.lisp (*sysdep-files*, *swank-files*): Don't load
swank/gray.lisp for ABCL.